Adjust usage message and add explicit help option.

Adjust usage message and add explicit help option.

Instead of dumping a huge splat on usage errors, output a brief message
instead with an option to request the full details.  Since we currently
only do short options, this option is -H (same as my own programs).

When requested by the user, program help text is not an error message
so it goes on standard output (that way it can be grepped and all that
good stuff).