Split settings out from syntax into proper files and add missing settings

Takes several non-syntax settings out of the syntax file and puts them in their standard files so vim will behave properly (e.g., users with filetype indent disabled will not have their indent settings overridden, and any users with syntax disabled would still have other settings applied).

Also adds standard guards for `b:did_ftplugin` and `b:did_indent`, and overrides a few previously-missing settings that ever ftplugin should override (formatoptions-=t, comments, commentstring).
Yep, it affects folding, and I think some vim plugins / snippet tools might use it for inserting comment characters. It's somewhat arbitrary and there's no way to have both.

The default of `/*%s*/` is also reasonable, so if you'd prefer to switch to that value or drop it from here entirely to stick with the default, I'm fine with that. (I just like setting it explicitly everywhere to set a precedent of considering it when you create new filetypes using existing ones for reference, since copypasta is so rampant in vim (and most filetypes happen to not be C).)