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/time/timer_create.c

@@ -51,8 +51,7 @@ static void install_handler()
 		.sa_flags = SA_SIGINFO | SA_RESTART
 	};
 	__libc_sigaction(SIGTIMER, &sa, 0);
-	sigaddset(&sa.sa_mask, SIGTIMER);
-	__libc_sigprocmask(SIG_UNBLOCK, &sa.sa_mask, 0);
+	__syscall(SYS_rt_sigprocmask, SIG_UNBLOCK, &SIGTIMER_SET, 0, 8);
 }
 
 static void *start(void *arg)