move `call_shell` to misc1.c
Move some fns to state.c
Move some fns to option.c
Move some fns to memline.c
Move `vim_chdir*` fns to file_search.c
Move some fns to new module, bytes.c
Move some fns to fileio.c

Fixes #2773

Problem:    Compiler warning for ignoring return value of fwrite().
Solution:   Do use the return value. (idea: Charles Campbell)

https://github.com/vim/vim/commit/285bf84b4b9aca828828a8729b04cd59ab333dac

Problem:    Compiler complains about ignoring return value of fwrite().
            (Michael Jarvis)
Solution:   Add (void).

https://github.com/vim/vim/commit/cf48767cd17130958a3076eed1872b6950947a0a

Regarding the individual items in the header:

`Vim - Vi improved	by Bram Moolenar`
  Bram Moolenar is already mentioned throughout the documentation, as
  well as the intro screen.

`:help uganda`
  It's already shown to all users who don't use `shortmess+=I` upon
  starting nvim, and is already placed prominently in help.txt, i.e.,
  `:help` run with no arguments.

`:help credits`
  Already mentioned near the top of help.txt.

`README.md`
  Already mentioned in develop.txt.

The former no longer exists in this repo; see the top of
src/nvim/README.md.

Refactor summary:
- extern int opcount --> extern long opcount
- bool find_decl(..., int len, ...) --> bool find_decl(..., size_t len, ...)
  * int find_ident_under_cursor(...) --> size_t find_ident_under_cursor(...)
    - int find_ident_at_pos(...) --> size_t find_ident_at_pos(...)
    - int modify_fname(..., int *usedlen, ..., int *fnamelen) --> int modify_fname(..., size_t *usedlen, ..., size_t *fnamelen)
      * char_u *eval_vars(..., int *usedlen, ...) --> char_u *eval_vars(..., size_t *usedlen, ...)
        - int find_cmdline_var(..., int *usedlen) --> ssize_t find_cmdline_var(..., size_t *usedlen)
        - static char_u *repl_cmdline(..., int srclen, ...) --> static char_u *repl_cmdline(..., size_t srclen, ...)
    - bool get_visual_text(..., int *lenp) --> bool get_visual_text(..., size_t *lenp)
      * char_u *find_file_name_in_path(..., int len, ...) --> char_u *find_file_name_in_path(..., size_t len, ...)
        - static char_u *eval_includeexpr(..., int len) --> static char_u *eval_includeexpr(..., size_t len)
        - char_u *find_file_in_path(..., int len, ...) --> char_u *find_file_in_path(..., size_t len, ...)
          * char_u *find_file_in_path_option(..., int len, ...) --> char_u *find_file_in_path_option(..., size_t len, ...)
            - char_u *find_directory_in_path(..., int len, ...) --> char_u *find_directory_in_path(..., size_t len, ...)
      * int spell_move_to(...) --> size_t spell_move_to(...)
        - int spell_check(...) --> size_t spell_check(...)
        - static int spell_bad_len --> static size_t spell_bad_len
    - void find_pattern_in_path(..., int len, ...) --> void find_pattern_in_path(..., size_t len, ...)

Helped-by: Justin M. Keyes <justinkz@gmail.com>