Userland et al: Pledge sigaction when needed

* In some cases, we can first call sigaction()/signal(), then *not* pledge
  sigaction.
* In other cases, we pledge sigaction at first, call sigaction()/signal()
  second, then pledge again, this time without sigaction.
* In yet other cases, we keep the sigaction pledge. I suppose these could all be
  migrated to drop it or not pledge it at all, if somebody is interested in
  doing that.

Services/SystemServer/main.cpp

@@ -105,7 +105,7 @@ static void mount_all_filesystems()
 
 int main(int, char**)
 {
-    if (pledge("stdio proc exec tty accept unix rpath wpath cpath chown fattr id", nullptr) < 0) {
+    if (pledge("stdio proc exec tty accept unix rpath wpath cpath chown fattr id sigaction", nullptr) < 0) {
         perror("pledge");
         return 1;
     }