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/ui-day.c

@@ -1067,7 +1067,7 @@ void ui_day_item_pipe(void)
 		return;
 
 	wins_prepare_external();
-	if ((pid = shell_exec(NULL, &pout, NULL, *arg, arg))) {
+	if ((pid = shell_exec(NULL, &pout, NULL, 0, *arg, arg))) {
 		fpout = fdopen(pout, "w");
 
 		switch (p->type) {