Shell: Kill existing jobs when exiting

Author: https://github.com/alimpfard
Commit: 143be7234f
Pull-request: https://github.com/SerenityOS/serenity/pull/2361
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Shell/Shell.h

@@ -162,6 +162,7 @@ private:
     };
 
     void cache_path();
+    void stop_all_jobs();
 
     IterationDecision wait_for_pid(const SpawnedProcess&, bool is_first_command_in_chain, int& return_value);