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/sigs.c

@@ -59,6 +59,7 @@ generic_hdlr (int sig)
         ;
       break;
     case SIGWINCH:
+      resize = 1;
       clearok (curscr, TRUE);
       (void)ungetch (KEY_RESIZE);
       break;