patch 9.1.1151: too many strlen() calls in getchar.c

patch 9.1.1151: too many strlen() calls in getchar.c

Problem:  too many strlen() calls in getchar.c
Solution: store last inserted and recorded lengths,
          add functions to retrieve those and use those
          functions (John Marriott)

closes: #16720

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