Show only a usage summary when -h is passed.

There was a long "usage" string contained within the program itself
that is now contained within the man page. The long usage would be
shown with "valtz -h", and a shorter one would be shown with simply
"valtz", which is an error by itself. This commit also unifies those
two: if you try to run valtz with no arguments, the "valtz -h" output
is shown but the exit code will be non-zero.