Shell: Put children in their own process groups and fix job control

This commit fixes job control by putting children in their own process
group, and proxying TTY signals to active jobs.
This also cleans up the code around builtin_disown a bit to use
the newer job interfaces.

Shell/Shell.h

@@ -111,6 +111,8 @@ public:
 
     u64 find_last_job_id() const;
     const Job* find_job(u64 id);
+    const Job* current_job() const { return m_current_job; }
+    void kill_job(const Job*, int sig);
 
     String get_history_path();
     void load_history();
@@ -163,6 +165,7 @@ private:
 
     void cache_path();
     void stop_all_jobs();
+    const Job* m_current_job { nullptr };
 
     virtual void custom_event(Core::CustomEvent&) override;