Consistency in xtt tool usage strings

[drbild]

The various usage strings in the xtt tool subcommands are not consistent:

• "Usage: xtt a b c"
• "Usage: xtt-tool a b c"
• "Usage: %s a b c"%argv[0]

We should standardize on one.

"Usage: %s a b c"%argv[0]" (at the top-level) or "Usage: %s %s a b c"%(argv[0], argv[1]) (within commands) seems correct to me.

Using argv ensures that usage string mirrors that name that the user typed.