everywhere: port everything to sigprocmask_many() and friends

This ports a lot of manual code over to sigprocmask_many() and friends.

Also, we now consistly check for sigprocmask() failures with
assert_se(), since the call cannot realistically fail unless there's a
programming error.

Also encloses a few sd_event_add_signal() calls with (void) when we
ignore the return values for it knowingly.

src/network/networkd.c

@@ -75,7 +75,7 @@ int main(int argc, char *argv[]) {
         if (r < 0)
                 goto out;
 
-        assert_se(sigprocmask_many(SIG_BLOCK, SIGTERM, SIGINT, -1) == 0);
+        assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGTERM, SIGINT, -1) >= 0);
 
         r = manager_new(&m);
         if (r < 0) {