AXFR support in net-dns is broken. This fixes it, and makes the
requisite modifications to enum_dns module. Basic problem is that AXFR
responses consist of a chain of DNS replies, not a single reply with
multiple answers. Previously, only the first of these replies, the SOA
record, was returned. Also added some exception handling to avoid
problems like #483.
Sometimes a nameserver won't have an A record for its own name. Check
for this and fall back to using the system resolver via
Rex::Socket.gethostbyname. Example:
$ dig +short zonetransfer.me NS
ns12.zoneedit.com.
ns16.zoneedit.com.
$ dig +short @ns12.zoneedit.com ns12.zoneedit.com A
$ dig +short @ns16.zoneedit.com ns12.zoneedit.com A
$ dig +short @ns16.zoneedit.com ns16.zoneedit.com A
$
Also removed an extra A lookup that was unnecessary.