patch 9.2.0291: too many strlen() calls

patch 9.2.0291: too many strlen() calls

Problem:  too many strlen() calls
Solution: refactor concat_fname() and remove calls to strlen()
          (John Marriott)

Function `concat_fnames()` can make up to 5 calls to `STRLEN()` (either
directly or indirectly via `STRCAT()`). In many cases the lengths of
arguments `fname1` and/or `fname2` are either known or can simply be
calculated.

This Commit refactors this function to accept the lengths of arguments
`fname1` and `fname2` as arguments. It also adds new argument `ret` to
return the resulting string as a `string_T`.

Additionally:
- function `add_pack_dir_to_rtp()` in `scriptfile.c`:
   Use a `string_T` to store local variables `new_rtp` and `afterdir`.
   Replace calls to `STRCAT()` with calls to `STRCPY()`.
   Change type of variable `keep` to `size_t` for consistency with
   other lengths.

- function `qf_get_fnum()` in `quickfix.c`:
  Use a `string_T` to store local variables `ptr` and `bufname`
- function `qf_push_dir()` in `quickfix.c`:
  Use a `string_T` to store local variable `dirname`.
  Replace call to `vim_strsave()` with `vim_strnsave()`.

- function `qf_guess_filepath()` in `quickfix.c`:
  Use a `string_T` to store local variable `fullname`.

- function `make_percent_swname()` in `memline.c`:
  Rename some variables to better reflect their use.
  Use a `string_T` to store local variables `d` and `fixed_name`.
  Slightly refactor to remove need to create an extra string.
- function `get_file_in_dir()` in `memline.c`:
  Use a `string_T` to store local variables `tail` and `retval`.
  Move some variables closer to where they are used.

- function `cs_resolve_file()` in `if_cscope.c`:
  Use a `string_T` to store local variable `csdir`.
  Remove one call to `STRLEN()`.

- function `add_pathsep()` in `filepath.c`:
  Refactor and remove 1 call to `STRLEN()`

- function `set_init_xdg_rtp()` in `option.c`:
  Use a `string_T` to store local variable `vimrc_xdg`.

closes: #19854

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