Call setsid() for hook/notification commands

We do not want hook or notification commands to interact with the
terminal in any way. Create a new session for them.

Addresses GitHub issue #326.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/wins.c

@@ -624,7 +624,7 @@ void wins_launch_external(const char *arg[])
 	int pid;
 
 	wins_prepare_external();
-	if ((pid = shell_exec(NULL, NULL, NULL, *arg, arg)))
+	if ((pid = shell_exec(NULL, NULL, NULL, 0, *arg, arg)))
 		child_wait(NULL, NULL, NULL, pid);
 	wins_unprepare_external();
 }