Replace grid.h in screen.h and screen.h in buffer.h with grid_defs.h

Mostly avoids including eval.h, ex_cmds2.h and ex_docmd.h in other
headers.

Remove unused includes in src/nvim/buffer.c|h using the IWYU library.
Yet another step towards #6371 and #549

Problem:    Get E685 and E931 if buffer reload is interrupted.
Solution:   Do not abort deleting a dummy buffer. (closes vim/vim#5361)
https://github.com/vim/vim/commit/a6e8f888e7fc31b8ab7233509254fb2e2fe4089f

Problem:    Not sufficient parenthesis in preprocessor macros.
Solution:   Add more parenthesis.
https://github.com/vim/vim/commit/9dac9b1751dd43c02470cc6a2aecaeea27abcc80

* refactor: format all C files under nvim
* refactor: disable formatting for Vim-owned files:
    * src/nvim/indent_c.c
    * src/nvim/regexp.c
    * src/nvim/regexp_nfa.c
    * src/nvim/testdir/samples/memfile_test.c

Problem:    Still using default option values after using ":badd +1".
Solution:   Find a window where options were set.  Don't set the window when
            using ":badd".
https://github.com/vim/vim/commit/89b693e5627715cde080c3580c7b641c9bf0c06a

fixes #6635
closes #9693