[krbdev.mit.edu #8376] add usage information for kdb5_util subcommands

[Karl-Philipp Richter via RT]

Invoking `kdb5_util -h` doesn't provide information about options of
subcommands like `create` (e.g. what does the `-s` do?). Those info
should be included in `--help` giving more information than `-h` or a
`-h/--help` command should be introduced for every subcommand. In case
those functions are already available, they should be mentioned in the
output of `kdb5_util -h`.

experienced with 0533ebf