the value placed in the aux vector AT_MINSIGSTKSZ by the kernel is
purely the signal frame size, and does not include any execution space
for the signal handler. this is contrary to the POSIX definition of
MINSIGSTKSZ to be a value that can actually execute at least some
minimal signal handler, and contrary to the historical definitions of
MINSIGSTKSZ which had at least 1k of headroom.

use the legacy constant values if the kernel does not provide
AT_MINSIGSTKSZ (__getauxval will return 0 in this case) and as a
safety check if something is wrong and the provided value is less than
the legacy constant.

sysconf(_SC_SIGSTKSZ) returns SIGSTKSZ adjusted for the difference
between the legacy constant MINSIGSTKSZ and the runtime value, so that
the working space the application has on top of the minimum remains
invariant under changes to the minimum.

as a result of ISA extensions exploding register file sizes on some
archs, using a constant for minimum signal stack size no longer seems
viably future-proof. add sysconf keys allowing the kernel to provide a
machine-dependent minimum applications can query to ensure they
allocate sufficient space for stacks. the key names and indices align
with the same functionality in glibc.

see commit d5a5045382315e36588ca225889baa36ed0ed38f for previous
action on this subject.

ultimately, the macros MINSIGSTKSZ and SIGSTKSZ probably need to be
deprecated, but that is standards-amendment work outside the scope of
a single implementation.

_CS_POSIX_V7_THREADS_CFLAGS and _CS_POSIX_V7_THREADS_LDFLAGS have been
missing for a long time, which is a conformance defect. we were
waiting on glibc to add them or at least agree on the numeric values
they will have so as to keep the numbering aligned. it looks like they
will be added to glibc with these numbers, and in any case, this list
does not have any significant churn that would result in the numbers
getting taken.

commit 5ce3737931bb411a8d167356d4d0287b53b0cbdc removed the inclusion
of libc.h from this file as spurious, but it's needed to get PAGE_SIZE
on archs where PAGE_SIZE is not a constant defined by limits.h.

libc.h was intended to be a header for access to global libc state and
related interfaces, but ended up included all over the place because
it was the way to get the weak_alias macro. most of the inclusions
removed here are places where weak_alias was needed. a few were
recently introduced for hidden. some go all the way back to when
libc.h defined CANCELPT_BEGIN and _END, and all (wrongly implemented)
cancellation points had to include it.

remaining spurious users are mostly callers of the LOCK/UNLOCK macros
and files that use the LFS64 macro to define the awful *64 aliases.

in a few places, new inclusion of libc.h is added because several
internal headers no longer implicitly include libc.h.

declarations for __lockfile and __unlockfile are moved from libc.h to
stdio_impl.h so that the latter does not need libc.h. putting them in
libc.h made no sense at all, since the macros in stdio_impl.h are
needed to use them correctly anyway.

commits leading up to this one have moved the vast majority of
libc-internal interface declarations to appropriate internal headers,
allowing them to be type-checked and setting the stage to limit their
visibility. the ones that have not yet been moved are mostly
namespace-protected aliases for standard/public interfaces, which
exist to facilitate implementing plain C functions in terms of POSIX
functionality, or C or POSIX functionality in terms of extensions that
are not standardized. some don't quite fit this description, but are
"internally public" interfacs between subsystems of libc.

rather than create a number of newly-named headers to declare these
functions, and having to add explicit include directives for them to
every source file where they're needed, I have introduced a method of
wrapping the corresponding public headers.

parallel to the public headers in $(srcdir)/include, we now have
wrappers in $(srcdir)/src/include that come earlier in the include
path order. they include the public header they're wrapping, then add
declarations for namespace-protected versions of the same interfaces
and any "internally public" interfaces for the subsystem they
correspond to.

along these lines, the wrapper for features.h is now responsible for
the definition of the hidden, weak, and weak_alias macros. this means
source files will no longer need to include any special headers to
access these features.

over time, it is my expectation that the scope of what is "internally
public" will expand, reducing the number of source files which need to
include *_impl.h and related headers down to those which are actually
implementing the corresponding subsystems, not just using them.

- REALTIME_SIGNALS is supposed to be version-valued
- DELAYTIMER_MAX was wrongly using the min allowed max
- unavailable compilation environments wrongly used 0 instead of -1

POSIX requires the symlink function to fail with ENAMETOOLONG if the
link contents to be written exceed SYMLINK_MAX in length, but neither
Linux nor our syscall wrapper code enforce this. the value 255 for
SYMLINK_MAX is not meaningful and does not seem to have been motivated
by anything except perhaps a wrong assumption that a definition was
mandatory. it has been present (though moving through bits to
top-level limits.h) since the beginning of the project history.

[f]pathconf is entitled to return -1 as the limit for conf names for
which there is no hard limit, with the usual POSIX note that an
indefinite limit does not imply an infinite limit. in principle we
might should report a limit for filesystems that impose one, but such
functionality is not currently present for any of the pathconf limits,
and adding it is beyond the scope of fixing the incorrect limit.

sysconf should return -1 for infinity, not LONG_MAX.