resolver: Actually set port in resulting ca's

This got broken in 2093fb5c `comm etc.: Provide comm_addr_equal'.

We mistakenly removed the code that copied the port from q, along with
the code that copied the adns answer from ra.  (The sockaddr that
comes back from adns obviously doesn't have a port number in it.)

As a result all actual DNS resolutions would result in an unuseable
sockaddr with port==0.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>