Problem: The default commentstring for C/C++ can lead to invalid code
when commenting and does not match the Nvim codebase.
Solution: Change commentstring to `// %s` as used by Neovim. Also
set all commentstrings that derive from the default C string explicitly
(and correctly).
The options 'path', 'include', and 'define' all use C-specific default
values. This may have made sense a long time ago when Vim was mostly
used just for writing C, but this is no longer the case, and we have
ample support for filetype specific configuration. Make the default
values of these options empty and move the C-specific values into a
filetype plugin where they belong.
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
