Avoid redundant redraws on resize

Use a global flag to record whether the terminal was resized instead of
redrawing everything each time a KEY_RESIZE is read.

Add some additional checks to help_write_pad() as invalid actions may be
passed now due to using signals instead of virtual key presses.

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

src/calcurse.h

@@ -864,6 +864,7 @@ void         psleep (unsigned);
 
 /* vars.c */
 extern int               col, row;
+extern int               resize;
 extern unsigned          colorize;
 extern enum ui_mode      ui_mode;
 extern int               days[12];