rename libcontainer/dmz to a better name

[cyphar]

(Suggested by @AkihiroSuda in #4509.)

Side note: it is confusing that the new overlay stuff is also called "dmz"

Let's not do that until the code will be stabilized (say until v1.3 release) as otherwise backporting stuff will be a pain.

Originally posted by @kolyshkin in #4509 (comment)