use a separate signal from SIGCANCEL for SIGEV_THREAD timers

otherwise we cannot support an application's desire to use
asynchronous cancellation within the callback function. this change
also slightly debloats pthread_create.c.

src/signal/sigaction.c

@@ -35,7 +35,7 @@ int __libc_sigaction(int sig, const struct sigaction *sa, struct sigaction *old)
 
 int __sigaction(int sig, const struct sigaction *sa, struct sigaction *old)
 {
-	if (sig == SIGCANCEL || sig == SIGSYSCALL) {
+	if (sig-SIGCANCEL < 3U) {
 		errno = EINVAL;
 		return -1;
 	}

src/signal/sigprocmask.c

@@ -22,6 +22,7 @@ int __sigprocmask(int how, const sigset_t *set, sigset_t *old)
 		set = &tmp;
 		sigdelset(&tmp, SIGCANCEL);
 		sigdelset(&tmp, SIGSYSCALL);
+		sigdelset(&tmp, SIGTIMER);
 	}
 	return __libc_sigprocmask(how, set, old);
 }

src/signal/sigrtmin.c

@@ -1,4 +1,4 @@
 int __libc_current_sigrtmin()
 {
-	return 34;
+	return 35;
 }