image: enable basic tab completion for bash

[citrus-it]

This gets basic tab completion working for bash builtins and for various illumos commands that take a zone name -- most usefully zlogin.

It also adds oxlog's location to the PATH in the GZ, where it's most often used.

[jclulow]

I will have a look at this! I think we could maybe do something with a small Rust program here to avoid all the bash logic.

[citrus-it]

I think we should merge the small fragment that is in this branch now to get into the next release, and we can circle back later with a better solution if there is one.

[jclulow]

With my apologies for the delay, I've pushed up what I came up with here: https://github.com/oxidecomputer/helios-bash-completion

Can you try it to see if it works for you?