Add vector of cleanup/termination events to be executed before quit

src/exec.cpp

@@ -359,6 +359,12 @@ static bool can_use_posix_spawn_for_job(const job_t *job, const process_t *proce
 void internal_exec(job_t *j, const io_chain_t &&all_ios) {
     // Do a regular launch -  but without forking first...
 
+    // since we are about to quit, make sure to run pending cleanup tasks
+    while (!before_exit.empty()) {
+        before_exit.top()();
+        before_exit.pop();
+    }
+
     // setup_child_process makes sure signals are properly set up.
 
     // PCA This is for handling exec. Passing all_ios here matches what fish 2.0.0 and 1.x did.