The `--help` option has been replaced by a new `help` subcommand.
Invoking `mpbb help` displays global usage and available subcommands,
while `mpbb help <subcommand>` displays subcommand-specific usage.
Each subcommand script `mpbb-FOO` must now define a function `FOO-usage`
that prints its specific help to standard out.
git-svn-id: https://svn.macports.org/repository/macports/contrib/mp-buildbot@152528 d073be05-634f-4543-b044-5fe20cf6d1d6
