Collect status from dead jobs and don't invoke the callback until both

all input (the socket is closed) and status is available.

tmux.h

@@ -567,6 +567,7 @@ ARRAY_DECL(keylist, int);
 struct job {
 	char		*cmd;
 	pid_t		 pid;
+	int		 status;
 
 	struct client	*client;
 
@@ -577,6 +578,9 @@ struct job {
 	void		(*freefn)(void *);
 	void		*data;
 
+	int		 flags;
+#define JOB_DONE 0x1
+
 	RB_ENTRY(job)	 entry;
 	SLIST_ENTRY(job) lentry;
 };