Allow skydns itself to use env variables

Reuse the variables defined in skydnsctl for skydns itself. Also rename
them slightly to keep the two much more in sync.

SKYDNS_DNSPORT -> SKYDNS_DNS (something like 127.0.0.1:1053) in stead of
    a number
SKYDNS_DNSDOMAIN -> SKYDNS_DOMAIN (a string like skydns.local)

The environment variable SKYDNS is used in skydns, the prefix http:// is
removed before use. This might confuse people.

Sadly this is a minor backward incompatible change, because
skydns.NewClient needed to be changed
If SKYDNS does not start with http add http://

This makes it possible to share SKYDNS between skydns and
skydnsctl.
DNS corrections

Give back NXDOMAIN when a service isn't found.
Also set the AA bit on replies (skydns is authoritative for this stuff)
and announce to be capable of doing recursion (RA bit).
Include SOA record in NXDOMAIN

The rules state NXDOMAIN requires a SOA record in the reply.
Make that happen.
Remove qtype == ANY here, already handled

This is handled in the 'if' above it. It can be removed here.
Send NODATA for unsupported record types
Make sure the correct bits are set when DNS forwarding
Merge pull request #61 from miekg/dns

Fix DNS replies
Merge pull request #60 from miekg/env

Allow skydns itself to use env variables