prompt streamlining, move disambiguate into hook

this way, we keep it all neatly in one place. works for all (both) of my
prompts, disambiguate is no longer autoloaded but simply defined in the
hook file, affecting both vcs_info enabled and regular directories.

also, disambiguate-keeplast is now simply an option to regular
disambiguate. boo code duplication!
prompt: outsource exitstatus signal names into hook