* Pick up perl executable from environment.

  * use warnings;

  * Make usage() a subroutine rather than a string.

  * Use @table_names in usage() to avoid confusion when the list of
    tables is updated (the 'ditag' table was missing from the list, but
    not in the old usage string).

  * Better usage() text.

  * Better information messages when running the script.

  * When unable to dump the old table, skip to the next database rather
    than dying.

  * Always use long option names when calling external programs from scripts!

  * Use do() rather than prepare(), execute(), finish() for simple
    things like these.