networkd: use OrderedSets instead of strvs to store lists of domains

networkd: use OrderedSets instead of strvs to store lists of domains

We were already using OrderedSets in the manager object, but strvs in the
configuration parsing code. Using sets gives us better scaling when many
domains are used.

In oss-fuzz #13059 the attached reproducer takes approximately 30.5 s to be
parsed. Converting to sets makes this go down to 10s. This is not _vastly_
faster, but using sets seems like a nicer approach anyway. In particular, we
avoid the quadratic de-unification operation after each addition.