sys_select: define sigset_t more portably

sys_select: define sigset_t more portably

* lib/sys_select.in.h: Always include <sys/types.h>, since
we now need sigset_t and mingw defines it there.
Include <signal.h> before split inclusion guard, to avoid
mishaps on Solaris, whose <signal.h> eventually includes us.
* m4/signal_h.m4 (gl_SIGNAL_H): Require gl_CHECK_TYPE_SIGSET_T.
(gl_CHECK_TYPE_SIGSET_T): New macro, most of the contents of
which come from ...
* m4/signalblocking.m4 (gl_SIGNALBLOCKING): Require
gl_CHECK_TYPE_SIGSET_T.
(gl_PREREQ_SIGPROCMASK): Now a no-op, since gl_CHECK_TYPE_SIGSET_T
does the real work.
* modules/sys_select (Depends-on): Add 'signal'.