[mapplauncherd] When trying to invoke an application fails, fall back to the generic booster.

Falling back to an unboosted exec should be the path of absolute last resort,
nothing more. Failing to do this can result in broken privileges, etc.

src/invoker/invoker.c

@@ -617,7 +617,20 @@ static int invoke(int prog_argc, char **prog_argv, char *prog_name,
         int fd = invoker_init(app_type);
         if (fd == -1)
         {
-            invoke_fallback(prog_argv, prog_argv[0], wait_term);
+            // if the attempt was to use the generic booster, and that failed,
+            // then give up and start unboosted. otherwise, make an attempt to
+            // use the generic booster before not boosting. this means single
+            // instance support (for instance) will still work.
+            if (strcmp(app_type, "generic") == 0)
+            {
+                warning("Can't try fall back to generic, already using it\n");
+                invoke_fallback(prog_argv, prog_argv[0], wait_term);
+            }
+            else
+            {
+                warning("Booster %s is not available. Falling back to generic.\n", app_type);
+                invoke(prog_argc, prog_argv, prog_argv[0], "generic", magic_options, wait_term, respawn_delay, test_mode);
+            }
         }
         // "normal" invoke through a socket connetion
         else