patch 9.1.1887: string handling in strings.c can be improved

patch 9.1.1887: string handling in strings.c can be improved

Problem:  string handling in strings.c can be improved
Solution: Refactor strings.c and remove calls to STRLEN()
          (John Marriott)

This change does:
- In vim_strsave_shellescape() a small cosmetic change.
- In string_count() move the call to STRLEN() outside the while loop.
- In blob_from_string() refactor to remove call to STRLEN().
- In string_from_blob() call vim_strnsave() instead of vim_strsave().
- In vim_snprintf_safelen() call vim_vsnprintf_typval() directly instead
  of vim_vsnprintf() which then calls vim_vsnprintf_typval().
- In copy_first_char_to_tv() change to return -1 on failure or the length
  of resulting v_string. Change string_filter_map() and string_reduce() to
  use the return value of copy_first_char_to_tv().

closes: #18617

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