resolved: move DNS class utilities to dns-type.c and add more helpers

resolved: move DNS class utilities to dns-type.c and add more helpers

Let's make DNS class helpers more like DNS type helpers, let's move them
from resolved-dns-rr.[ch] into dns-type.[ch].

This also adds two new calls dns_class_is_pseudo() and
dns_class_is_valid_rr() which operate similar to dns_type_is_pseudo()
and dns_type_is_valid_rr() but for classes instead of types.

This should hopefully make handling of DNS classes and DNS types more
alike.