patch 9.1.1440: too many strlen() calls in os_win32.c

patch 9.1.1440: too many strlen() calls in os_win32.c

Problem:  too many strlen() calls in os_win32.c
Solution: refactor code and remove calls to strlen()
          (John Marriott)

The following changes have been made:
- In mch_init_g():
  - refactor to remove calls to STRLEN().
  - use vim_strnsave() instead of vim_strsave().
  - set a flag if vimrun_path is stored in allocated memory so it can be
     freed at exit.
- In mch_exit() free vimrun_path if it was stored in allocated memory.
- In fname_case() make a small optimisation by measuring the length of
  name only if needed.
- In copy_extattr() make a small optimisation by replacing call to
  STRCAT() with STRCPY().

closes: #17462

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