New module sigaction, for mingw.

New module sigaction, for mingw.

* modules/sigaction: New module...
* modules/sigaction-tests: ...and its test.
* m4/sigaction.m4: New file.
* lib/sigaction.c: Likewise.
* tests/test-sigaction.c: Likewise.
* m4/signal_h.m4 (gl_SIGNAL_H_DEFAULTS): Add sigaction variables.
* modules/signal (Makefile.am): Likewise.
* lib/signal.in.h (!@HAVE_SIGACTION@): Define replacements when
needed.
* doc/posix-headers/signal.texi (signal.h): Mention provided
types.
* doc/posix-functions/siginterrupt.texi (siginterrupt): Mention
that sigaction is preferable.
* doc/posix-functions/sigaction.texi (sigaction): Mention new
module.
* MODULES.html.sh (Support for systems lacking POSIX:2001): Add
sigaction.

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