setenv: relax requirement in light of POSIX ruling

setenv: relax requirement in light of POSIX ruling

Requiring that {un,}setenv gracefully reject NULL is just a waste
of processing power; POSIX agreed to this argument in
 http://austingroupbugs.net/view.php?id=185
so we no longer worry whether a native implementation handles NULL.

* m4/setenv.m4 (gl_FUNC_SETENV_SEPARATE): Test handling of "" but
not NULL.
* tests/test-setenv.c (main): Relax test.
* tests/test-unsetenv.c (main): Likewise.
* doc/posix-functions/setenv.texi (setenv): Document this.
* doc/posix-functions/unsetenv.texi (unsetenv): Likewise.

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