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/test/test-pty.c

@@ -129,7 +129,7 @@ int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
 
-        assert_se(sigprocmask_many(SIG_BLOCK, SIGCHLD, -1) >= 0);
+        assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGCHLD, -1) >= 0);
 
         /* Oh, there're ugly races in the TTY layer regarding HUP vs IN. Turns
          * out they appear only 10% of the time. I fixed all of them and

src/test/test-udev.c

@@ -116,7 +116,7 @@ int main(int argc, char *argv[]) {
 
         event = udev_event_new(dev);
 
-        assert_se(sigprocmask_many(SIG_BLOCK, SIGTERM, SIGINT, SIGHUP, SIGCHLD, -1) == 0);
+        assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGTERM, SIGINT, SIGHUP, SIGCHLD, -1) >= 0);
 
         /* do what devtmpfs usually provides us */
         if (udev_device_get_devnode(dev) != NULL) {