bsdinstall: jail: Fix DISTMENU items

[jlduran]

Summary

The menu was incorrectly using the fourth column (distname) instead of
the first (dist) of the MANIFEST.

The actual file name is on the first column of the MANIFEST file.
Remove the .txz part of the name to build the menu options.

Test Plan

1. Create a jail using bsdinstall:

# mkdir -p /jail/dists
# bsdinstall jail /jail/dists

2. Choose "Distribution Sets".
3. Choose "lib32-dbg" (without this patch, it will show up as "lib32_dbg" and fail).

[jlduran]

Thinking about it, I probably won't probably come back to work on bsdinstall.
Apply a minimal fix.

[jlduran]

• We are not looking for POSIX comliance. A few other scripts are also using double equals for comparison, let's not change that for now.
• Update the description to better reflect that without this patch any selected set that contains a hyphen will fail to download.

[jamie]

It only touches peripherally on jails, but sure, looks good. One could ask why we decided to have a separate and subtlely different "dist" and "distname" but I suppose that's water long since under the bridge.

[jlduran]

In D53177#1216754, @jamie wrote:

It only touches peripherally on jails, but sure, looks good. One could ask why we decided to have a separate and subtlely different "dist" and "distname" but I suppose that's water long since under the bridge.

Thank you!