gnulib-common: prefer _GL_UNUSED over _UNUSED_PARAMETER_

gnulib-common: prefer _GL_UNUSED over _UNUSED_PARAMETER_

There are more contexts where __attribute__((__unused__)) is
useful than just parameter lists.  Also, naming the macro
_GL_UNUSED fits with the recent addition of _GL_ARG_NONNULL.
Preserve the name _UNUSED_PARAMETER_ for backwards-compatible
use in external projects.

* m4/gnulib-common.m4 (gl_COMMON): Create a more-appropriately
named alias for __attribute__((__unused__)).
* lib/chown.c: Update client.
* lib/fchmodat.c: Likewise.
* lib/fts.c: Likewise.
* lib/getdate.y: Likewise.
* lib/getgroups.c: Likewise.
* lib/getopt.c: Likewise.
* lib/getugroups.c: Likewise.
* lib/mkdir.c: Likewise.
* lib/mkfifo.c: Likewise.
* lib/mkfifoat.c: Likewise.
* lib/mknod.c: Likewise.
* lib/mknodat.c: Likewise.
* lib/readlink.c: Likewise.
* lib/se-context.in.h: Likewise.
* lib/se-selinux.in.h: Likewise.
* lib/sockets.c: Likewise.
* lib/symlink.c: Likewise.
* lib/symlinkat.c: Likewise.
* lib/unicodeio.c: Likewise.
* lib/unistr.h: Likewise.
* tests/test-areadlink.c: Likewise.
* tests/test-areadlinkat.c: Likewise.
* tests/test-filenamecat.c: Likewise.
* tests/test-fseeko.c: Likewise.
* tests/test-ftello.c: Likewise.
* tests/test-getdate.c: Likewise.
* tests/test-getgroups.c: Likewise.
* tests/test-gethostname.c: Likewise.
* tests/test-quotearg.c: Likewise.
* tests/test-version-etc.c: Likewise.
* tests/test-xalloc-die.c: Likewise.
* tests/test-xfprintf-posix.c: Likewise.
* tests/test-xprintf-posix.c: Likewise.
* tests/test-xvasprintf.c: Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>