posix: Fix getpwnam_r usage (BZ #1062)

This patch fixes longstanding misuse of errno after getpwnam_r,
which returns an error number rather than setting errno.  This is
sync with gnulib commit 5db9301.

Checked on x86_64-linux-gnu and on a build using build-many-glibcs.py
for all major architectures.

	[BZ #1062]
	* posix/glob.c (glob): Port recent patches to platforms
	lacking getpwnam_r.
	(glob): Fix longstanding misuse of errno after getpwnam_r, which
	returns an error number rather than setting errno.