overhaul implementation-internal signal protections

the new approach relies on the fact that the only ways to create
sigset_t objects without invoking UB are to use the sig*set()
functions, or from the masks returned by sigprocmask, sigaction, etc.
or in the ucontext_t argument to a signal handler. thus, as long as
sigfillset and sigaddset avoid adding the "protected" signals, there
is no way the application will ever obtain a sigset_t including these
bits, and thus no need to add the overhead of checking/clearing them
when sigprocmask or sigaction is called.

note that the old code actually *failed* to remove the bits from
sa_mask when sigaction was called.

the new implementations are also significantly smaller, simpler, and
faster due to ignoring the useless "GNU HURD signals" 65-1024, which
are not used and, if there's any sanity in the world, never will be
used.

src/signal/sigaddset.c

@@ -4,7 +4,7 @@
 int sigaddset(sigset_t *set, int sig)
 {
 	unsigned s = sig-1;
-	if (s >= 8*sizeof(sigset_t)) {
+	if (s >= 8*sizeof(sigset_t) || s-32U<3) {
 		errno = EINVAL;
 		return -1;
 	}