Add a read-only option

We don't save any configuration nor items if this is set. This should be
used with care, and hence there's no short option for this.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -946,6 +946,7 @@ extern int               resize;
 extern unsigned          colorize;
 extern int               foreground, background;
 extern enum ui_mode      ui_mode;
+extern int               read_only;
 extern int               days[12];
 extern char             *monthnames[12];
 extern char             *daynames[8];