patch 9.1.1050: too many strlen() calls in os_unix.c

Problem:  too many strlen() calls in os_unix.c
Solution: refactor os_unix.c and remove calls to strlen()
          (John Marriott)

closes: #16496

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

Author: John Marriott