Resolver: improved PTR response processing.

Resolver: improved PTR response processing.

The previous code only parsed the first answer, without checking its
type, and required a compressed RR name.

The new code checks the RR type, supports responses with multiple
answers, and doesn't require the RR name to be compressed.

This has a side effect in limited support of CNAME.  If a response
includes both CNAME and PTR RRs, like when recursion is enabled on
the server, PTR RR is handled.

Full CNAME support in PTR response is not implemented in this change.