Reimplement pants help.

Previously there were several different help paths,
help messages weren't uniform and in some cases were messy.

This change:

- Gets rid of the 'help' command.
- Gets rid of the 'help' goal.
- Has a single entry point to help message printing.
- Cleans up help messages. For example, we no longer print
  the list of goals unless you explicitly ask for it,
  because that list is long now.

Note that this gets rid of help for non-goal commands, but
those are going away soon anyway.

This change is in preparation for getting us off optparse and
onto our own custom cmd-line parser. We no longer use optparse's
help messages (but we do still use optparse as an implementation
detail for formatting flag help, for now).