refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

Problem:    Cannot lock a variable in legacy Vim script like in Vim9.
Solution:   Make ":lockvar 0" work.

https://github.com/vim/vim/commit/a187c43cfe8863d48b2159d695fedcb71f8525c1

Co-authored-by: Bram Moolenaar <Bram@vim.org>

refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

Work on https://github.com/neovim/neovim/issues/459

Work on https://github.com/neovim/neovim/issues/459

Problem:    Various white space and cosmetic mistakes.
Solution:   Change spaces to tabs, improve comments.

https://github.com/vim/vim/commit/6ed545e79735f23ff8e650bc2f0967e5a0baedc9

Co-authored-by: Bram Moolenaar <Bram@vim.org>

Also add the EXITFREE definition to main_lib rather than the nvim target, as the header generation needs the EXITFREE flag to work properly.

Problem:    Code is indented more than necessary.
Solution:   Use an early return where it makes sense. (Yegappan Lakshmanan,
            closes vim/vim#11813)

https://github.com/vim/vim/commit/e8575988969579f9e1439181ae338b2ff74054a8

Partial port as this depends on some previous eval and 'smoothscroll'
patches.

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

Fix remaining clint errors and remove error suppression completely.

Rename the lint targets to align with the established naming convention:

- lintc-clint lints with clint.py.
- lintc-uncrustify lints with uncrustify.
- lintc runs both targets.

lintc is also provided as a make target for convenience.

After this change we can remove these files:
https://github.com/neovim/doc/tree/gh-pages/reports/clint
https://github.com/neovim/doc/blob/main/ci/clint-errors.sh