based on a patch submitted by Kristian L. <email@thexception.net>

on old kernels, there's no way to detect errors; we must assume
negative syscall return values are pgrp ids. but if the F_GETOWN_EX
fcntl works, we can get a reliable answer.

The long double adjustment was wrong:
The usual check is
  mant_bits & 0x7ff == 0x400
before doing a mant_bits++ or mant_bits-- adjustment since
this is the only case when rounding an inexact ld80 into
double can go wrong. (only in nearest rounding mode)

After such a check the ++ and -- is ok (the mantissa will end
in 0x401 or 0x3ff).

fma is a bit different (we need to add 3 numbers with correct
rounding: hi_xy + lo_xy + z so we should survive two roundings
at different places without precision loss)

The adjustment in fma only checks for zero low bits
  mant_bits & 0x3ff == 0
this way the adjusted value is correct when rounded to
double or *less* precision.
(this is an important piece in the fma puzzle)

Unfortunately in this case the -- is not a correct adjustment
because mant_bits might underflow so further checks are needed
and this was the source of the bug.

unicode char data has both "W" and "F" wide types and the old table
only included the "W" ones. this omitted U+3000 (ideographic space)
and all the wide-ascii, etc.

this is silly, but it makes apps that read binary junk and interpret
it as ld80 "safer", and it gets gnulib to stop replacing printf...

it should return the error code rather than 0/-1 and setting errno.

at the point pclose might receive and act on cancellation, it has
already invalidated the FILE passed to it. thus, per musl's QOI
guarantees about cancellation and resource allocation/deallocation,
it's not a candidate for cancellation.

if it were required to be a cancellation point by posix, we would have
to switch the order of deallocation, but somehow still close the pipe
in order to trigger the child process to exit. i looked into doing
this, but the logic gets ugly, and i'm not sure the semantics are
conformant, so i'd rather just leave it alone unless there's a need to
change it.

close was the only cancellation point called from popen, but it left
popen with major resource leaks if any call to close got cancelled.
the easiest, cheapest fix is just to use a non-cancellable close
function.