Return post-insert loc in all cases.

We've got it, no point in throwing it away.
Wired up auto-fill (for text mode).

Also starting on fill-paragraph &c.
Doc tweak.
Revamped auto-fill to be more easily customized.
Customize auto-fill in code mode:

- only auto-fill when we're inside a line comment or doc comment
- prepend a comment prefix to the auto-wrapped line, and reindent

Also nixed some use of protected. Modes are generally not passed around by
reference, so whether something is public or protected is not super important.
Either something is public and derived modes might override and jiggle with it,
or it's private and they can keep their grubby hands off.