src/: Carve out a `representation' subfield of adns_rrtype.

Only the bits covered by adns_rrt_reprmask are used in translating a
query type into a typeinfo structure.

Arrange for adnstest to only print out the representation bits of the
type; otherwise lots of the regression test files need to change for no
especially good reason.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>