patch 9.1.0824: too many strlen() calls in register.c

patch 9.1.0824: too many strlen() calls in register.c

Problem:  too many strlen() calls in register.c
Solution: refactor code, add string_T struct to keep track
          of string lengths (John Marriott)

closes: #15952

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