pthread_sigmask: new module

pthread_sigmask: new module

* MODULES.html.sh (Support for systems lacking POSIX:2008): Add it.
* doc/posix-functions/pthread_sigmask.texi: Document new module.
* lib/signal.in.h (pthread_sigmask): Arrange for replacement.
This is done only as a macro; I don't know how well that'll
work for C++.  Move <sys/types.h> include before the include_next,
to avoid mishap on Solaris.
* m4/signal_h.m4 (gl_SIGNAL_H, gl_SIGNAL_H_DEFAULTS): Check for it.
* modules/signal (Makefile.am): Substitute the check's results.
* modules/pthread_sigmask, m4/pthread_sigmask.m4: New files.