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.

Applications/Debugger/main.cpp

@@ -173,7 +173,7 @@ void print_help()
 
 int main(int argc, char** argv)
 {
-    if (pledge("stdio proc exec rpath tty", nullptr) < 0) {
+    if (pledge("stdio proc exec rpath tty sigaction", nullptr) < 0) {
         perror("pledge");
         return 1;
     }

Applications/FileManager/main.cpp

@@ -67,7 +67,7 @@ static int run_in_windowed_mode(RefPtr<Core::ConfigFile>, String initial_locatio
 
 int main(int argc, char** argv)
 {
-    if (pledge("stdio thread shared_buffer accept unix cpath rpath wpath fattr proc exec", nullptr) < 0) {
+    if (pledge("stdio thread shared_buffer accept unix cpath rpath wpath fattr proc exec sigaction", nullptr) < 0) {
         perror("pledge");
         return 1;
     }

Applications/Terminal/main.cpp

@@ -179,7 +179,7 @@ RefPtr<GUI::Window> create_settings_window(TerminalWidget& terminal)
 
 int main(int argc, char** argv)
 {
-    if (pledge("stdio tty rpath accept cpath wpath shared_buffer proc exec unix fattr", nullptr) < 0) {
+    if (pledge("stdio tty rpath accept cpath wpath shared_buffer proc exec unix fattr sigaction", nullptr) < 0) {
         perror("pledge");
         return 1;
     }