patch 9.1.0997: too many strlen() calls in drawscreen.c

patch 9.1.0997: too many strlen() calls in drawscreen.c

Problem:  too many strlen() calls in drawscreen.c
Solution: refactor drawscreen.c and remove calls to strlen(),
          make get_keymap_str() (in screen.c) return string length
          instead of TRUE/FALSE (John Marriott).

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>