Add command line option to suppress dialogs

Implement a -q/--quiet command line option to disable system dialogs
temporarily.

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

src/vars.c

@@ -61,6 +61,9 @@ enum ui_mode ui_mode = UI_CMDLINE;
 /* Don't save anything if this is set. */
 int read_only = 0;
 
+/* Hide system dialogs if set. */
+int quiet = 0;
+
 /* Applications can trigger a reload by sending SIGUSR1. */
 int want_reload = 0;