signal, spawn: use link warnings

signal, spawn: use link warnings

These modules depended on link-warning, then didn't use it.

* lib/signal.in.h (sigset_t): Make unconditional.
(sigismember, sigemptyset, sigaddset, sigdelset, sigfillset)
(sigpending, sigprocmask, sigaction): Add link warnings.
* lib/spawn.in.h (posix_spawn, posix_spawnp, posix_spawnattr_init)
(posix_spawnattr_destroy, posix_spawnattr_getsigdefault)
(posix_spawnattr_setsigdefault, posix_spawnattr_getsigmask)
(posix_spawnattr_setsigmask, posix_spawnattr_getflags)
(posix_spawnattr_setflags, posix_spawnattr_getpgroup)
(posix_spawnattr_setpgroup, posix_spawnattr_getschedpolicy)
(posix_spawnattr_setschedpolicy, posix_spawnattr_getschedparam)
(posix_spawnattr_setschedparam, posix_spawn_file_actions_init)
(posix_spawn_file_actions_destroy)
(posix_spawn_file_actions_addopen)
(posix_spawn_file_actions_addclose)
(posix_spawwn_file_actions_adddup2): Likewise.
* m4/signal_h.m4 (gl_SIGNAL_H): Guarantee uid_t.
* tests/test-signal.c (main): Enhance test.

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