refactor: replace char_u with char

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

refactor: replace char_u with char

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

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

refactor: replace char_u with char

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

refactor: replace char_u with char

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

Use "0" for 'foldcolumn' when w_p_fdc_save is empty, like how
"manual" is used for 'foldmethod' when w_p_fdm_save is empty.

Problem:    Code uses too much indent.
Solution:   Use an early return. (Yegappan Lakshmanan, closes vim/vim#11747)

https://github.com/vim/vim/commit/465de3a57b815f1188c707e7c083950c81652536

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

Problem: With two files open side by side in diff mode and deleting all of the contents of one file, the w_topfill value, which indicates the number of filler lines at the top of the window is set to be a negative number, and it will result in the virtual_lines_passed variable also being negative, and this while loop will run when it shouldn't. While calculating where the cursor and topline should be with linematch enabled, this topfill value is used to put the cursor and top line where it should be in the other windows. If that topfill number is negative, this causes a segfault. 

Solution: Check for positive top fill.

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