Merge the sigaction() and sigprocmask() overloads/wrappers from libpthread

into libc, and move pthread_sigmask() as well (just a trivial wrapper).
This provides consistent handling of SIGTHR between single- and multi-threaded
programs and is a step in the merge of all the libpthread overloads, providing
some ASM and Makefile bits that the other wrappers will need.
ok deraadt@ millert@

src/lib/libc/stdlib/abort.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: abort.c,v 1.18 2015/09/13 08:31:47 guenther Exp $ */
+/*	$OpenBSD: abort.c,v 1.19 2015/10/23 04:39:24 guenther Exp $ */
 /*
  * Copyright (c) 1985 Regents of the University of California.
  * All rights reserved.
@@ -34,7 +34,6 @@
 #include "thread_private.h"
 #include "atexit.h"
 
-int	_thread_sys_sigprocmask(int, const sigset_t *, sigset_t *);
 
 void
 abort(void)
@@ -48,7 +47,7 @@ abort(void)
 	 * any errors -- X311J doesn't allow abort to return anyway.
 	 */
 	sigdelset(&mask, SIGABRT);
-	(void)_thread_sys_sigprocmask(SIG_SETMASK, &mask, (sigset_t *)NULL);
+	(void)sigprocmask(SIG_SETMASK, &mask, NULL);
 
 	(void)raise(SIGABRT);
 
@@ -57,7 +56,7 @@ abort(void)
 	 * it again, only harder.
 	 */
 	(void)signal(SIGABRT, SIG_DFL);
-	(void)_thread_sys_sigprocmask(SIG_SETMASK, &mask, (sigset_t *)NULL);
+	(void)sigprocmask(SIG_SETMASK, &mask, NULL);
 	(void)raise(SIGABRT);
 	_exit(1);
 }