Ignore signals during command execution

Disable signal handlers in wins_prepare_external() and reactivate them
in wins_unprepare_external().

Before, it was possible that resizing the window during editor/pager
mode resulted in the calcurse main screen appearing on top.

Addresses BUG#9.

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

src/calcurse.h

@@ -883,6 +883,8 @@ void recur_apoint_paste_item(void);
 /* sigs.c */
 void sigs_init(void);
 unsigned sigs_set_hdlr(int, void (*)(int));
+void sigs_ignore(void);
+void sigs_unignore(void);
 
 /* todo.c */
 extern llist_t todolist;