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/vars.c

@@ -43,6 +43,7 @@
  * variables to store window size
  */
 int col = 0, row = 0;
+int resize = 0;
 
 /* variable to tell if the terminal supports color */
 unsigned colorize = 0;