refactor: replace char_u with char

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

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:    Crash when doing crazy things with hidden buffers.
Solution:   Dynamically allocate the list of autocommand windows.

Problem:    Using one window for executing autocommands is insufficient.
Solution:   Use up to five windows for executing autocommands.

https://github.com/vim/vim/commit/e76062c078debed0df818f70e4db14ad7a7cb53a

N/A patches for version.c:

vim-patch:9.0.0966: some compilers don't allow a declaration after a label

Problem:    Some compilers don't allow a declaration after a label.
Solution:   Move the declaration to the start of the block. (John Marriott)

https://github.com/vim/vim/commit/f86490ed4fdab213a28f667abd055c023a73d645

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

Cherry-picked from Vim patch 8.1.1612.

Problem:    deletebufline() may move marks in the wrong window.
Solution:   Find a window for the buffer being changed. (closes vim/vim#11583)

https://github.com/vim/vim/commit/228e422855d43965f2c3319ff0cdc26ea422c10f

Cherry-pick code change from patch 9.0.0961.

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

Problem:    Setting lines in another buffer may not work well.
Solution:   Make sure the buffer being changed has a window. (issue vim/vim#11558)

https://github.com/vim/vim/commit/c934bfa1b765505e5fc491f2ee7cc106894cafc8

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

Problem:    Some source files are too big.
Solution:   Move buffer and window related functions to evalbuffer.c and
            evalwindow.c. (Yegappan Lakshmanan, closes vim/vim#4898)

https://github.com/vim/vim/commit/261f346f8154c0ec7094a4a211c653c74e9f7c2e