Use sigaction module rather than signal().

Use sigaction module rather than signal().

* modules/c-stack (Depends-on): Add sigaction.
* modules/fatal-signal (Depends-on): Likewise.
* modules/nanosleep (Depends-on): Likewise.
* modules/sigprocmask (Files): Add sig-handler.h.
* modules/sigaction (Files): Likewise.
* lib/sig-handler.h (get_handler): New file, suggested by Paul
Eggert.
* lib/c-stack.c (SIGACTION_WORKS): Simplify conditions.
(c_stack_action) [!SIGACTION_WORKS]: Use sigaction, not signal.
* lib/fatal-signal.c (uninstall_handlers, install_handlers)
(init_fatal_signals): Likewise.
* lib/nanosleep.c (rpl_nanosleep): Likewise.
(siginterrupt): Delete fallback.
* lib/sigprocmask.c (handler_t): Delete.
(old_handlers): Use sa_handler_t instead.
* m4/nanosleep.m4 (gl_PREREQ_NANOSLEEP): Drop check for
siginterrupt.

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