Clink improvements

[chrisant996]

Ugh. Sorry for how the commits look.

I made changes in master, then backed them out and made them in a topic branch. But it's listing the master changes, the master backouts, and the topic branch changes.

Anyway, this is really just the final two changes:

1. 09a3d42: Update the z.cmd flag completions for Clink, add descriptions for them, and add directory completions for the -x flag.
2. d38e4dc: The z.lua script for initializing z.cmd (when missing) was missing two initial lines that are important for proper execution; the change adds them so that z.lua can fully recreate z.cmd if it's missing.