patch 9.1.0882: too many strlen() calls in insexpand.c

patch 9.1.0882: too many strlen() calls in insexpand.c

Problem:  too many strlen() calls in insexpand.c
Solution: Refactor insexpand.c and reduce number of calls to STRLEN(),
          fix a warning get_next_filename_completion(), add new function
          ins_compl_leader_len() (John Marriott)

closes: #16095

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