[th/ifcfg-unset-well-known-keys]

add code to detect well-known ifcfg-rh keys (variable names).

This has lots of potential, for example we could warn about unknown keys and augment it with more meta-data. It's a small step the treat ifcfg a bit more generic.

Also, keep track of which variables we set, and delete all well-known, untouched (dirty) variables when writing the ifcfg-rh file.