Use an explicit job state instead of avoid closing our side of the

socketpair and setting it to -1 to mark when the other side is
closed. This avoids closing it while the libevent bufferevent still has
it (it could try to add it to the polled set which some mechanisms don't
like). Fixes part a problem reported by Bruno Sutic.

tmux.h

@@ -714,6 +714,12 @@ struct options {
 
 /* Scheduled job. */
 struct job {
+	enum {
+		JOB_RUNNING,
+		JOB_DEAD,
+		JOB_CLOSED
+	} state;
+
 	char		*cmd;
 	pid_t		 pid;
 	int		 status;