Adjust to recent Gnulib changes

Adjust to recent Gnulib changes

The latest Gnulib merge brought in free-posix, which causes 'free'
to preserve errno.  This lets us simplify some Emacs code that
calls 'free'.
* admin/merge-gnulib (GNULIB_MODULES): Add free-posix.
This module is pulled in by canonicalize-lgpl anyway,
so we might as well rely on it.
* lib-src/emacsclient.c (get_current_dir_name):
Sync better with src/sysdep.c.
* lib-src/etags.c (process_file_name, etags_mktmp):
* lib-src/update-game-score.c (unlock_file):
* src/fileio.c (file_accessible_directory_p):
* src/sysdep.c (get_current_dir_name_or_unreachable):
Simplify by assuming that 'free' preserves errno.
* src/alloc.c (malloc_unblock_input):
Preserve errno, so that xfree preserves errno.
* src/sysdep.c (get_current_dir_name_or_unreachable):
Simplify by using strdup instead of malloc+memcpy.
No need for realloc (and the old code leaked memory anyway on
failure); just use free+malloc.