bug#48201: bug(s) in underline-region

[Boruch Baum]

The functions of package underline region doesn't check whether it's
operations will work in the user's terminal or GUI environment. When I
try using the feature in emacs-nox, it replaces the region with
what is for that environment interleaved garbage in the form of "_\b".

I'm not sure what is the scope of the intent of the package, but for
non-persistent use, it could use overlays with an underline face.

At the very least, the feature should check the environment, and abort
upon detecting incompatability.

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