Path-mangling logic was failing to account for paths containing mentions

Path-mangling logic was failing to account for paths containing mentions
of '.' or '..'.  Extend canonicalize_path() to trim off trailing occurrences
of these things, and use it to fix up paths where needed (which I think is
only after places where we trim the last path component, but maybe some
others will turn up).  Fixes Josh's complaint that './initdb' does not
work.