* sub_joining_lines: Optimization for :%s/\n//
* sub_grow_buf: Allocation of buffer to contain replacement text
* sub_parse_flags: Parse {flags} from :s command into subflags_T

Although this doesn't reduce do_sub's size enough to satisfy lint, it
covers the more straightforward pieces.

Problem:    Recursive call to substitute gets stuck in sandbox. (Nikolai
            Pavlov)
Solution:   Handle the recursive call. (Christian Brabandt, closes vim/vim#950)
            Add a test.

https://github.com/vim/vim/commit/f5a39447a8ebe162ee62caa2ee502cd0e65eecaa

Closes #5118

Problem:    Equivalence classes are not properly tested.
Solution:   Add tests for multi-byte and latin1. Fix an error. (Owen Leibman)

https://github.com/vim/vim/commit/22e421549d54147d003f6444de007cb1d73f1d27

src/regexp.c changes weren't applied because they're specific to EBCDIC
handling, which has been dropped from nvim.

The latin1-specific tests were also removed since neovim intends to
remove the ability to have 'encoding' set to anything other than utf8.

This makes stderr and exit callbacks work for rpc jobs

Windows: path_is_absolute()

Check if drive letter is alphabetic character in
path_is_absolute_path().