patch by Jens Gustedt. previously, the intended policy was to use __environ in code that must conform to the ISO C namespace requirements, and environ elsewhere. this policy was not followed in practice anyway, making things confusing. on top of that, Jens reported that certain combinations of link-time optimization options were breaking with the inconsistent references; this seems to be a compiler or linker bug, but having it go away is a nice side effect of the changes made here.
this avoids duplicating the fragile logic for executing an external program without fork.
read should never return anything but 0 or sizeof ec here, but if it does, we want to treat any other return as "success". then the caller will get back the pid and is responsible for waiting on it when it immediately exits.
the proposed change was described in detail in detail previously on the mailing list. in short, vfork is unsafe because: 1. the compiler could make optimizations that cause the child to clobber the parent's local vars. 2. strace is buggy and allows the vforking parent to run before the child execs when run under strace. the new design uses a close-on-exec pipe instead of vfork semantics to synchronize the parent and child so that the parent does not return before the child has finished using its arguments (and now, also its stack). this also allows reporting exec failures to the caller instead of giving the caller a child that mysteriously exits with status 127 on exec error. basic testing has been performed on both the success and failure code paths. further testing should be done.
linux's sched_* syscalls actually implement the TPS (thread scheduling) functionality, not the PS (process scheduling) functionality which the sched_* functions are supposed to have. omitting support for the PS option (and having the sched_* interfaces fail with ENOSYS rather than omitting them, since some broken software assumes they exist) seems to be the only conforming way to do this on linux.
this mirrors the stdio_impl.h cleanup. one header which is not strictly needed, errno.h, is left in pthread_impl.h, because since pthread functions return their error codes rather than using errno, nearly every single pthread function needs the errno constants. in a few places, rather than bringing in string.h to use memset, the memset was replaced by direct assignment. this seems to generate much better code anyway, and makes many functions which were previously non-leaf functions into leaf functions (possibly eliminating a great deal of bloat on some platforms where non-leaf functions require ugly prologue and/or epilogue).
ideally, system would also be cancellable while running the external command, but I cannot find any way to make that work without either leaking zombie processes or introducing behavior that is far outside what the standard specifies. glibc handles cancellation by killing the child process with SIGKILL, but this could be unsafe in that it could leave the data being manipulated by the command in an inconsistent state.
__release_ptc() is only valid in the parent; if it's performed in the child, the lock will be unlocked early then double-unlocked later, corrupting the lock state.