Drop laddr argument from Dial.

Drop cname return from LookupHost.

Add LookupIP, LookupCNAME, ParseCIDR, IP.Equal.
Export SplitHostPort, JoinHostPort.
Add AAAA (IPv6) support to host lookups.

Preparations for implementing some of the
lookups using cgo.

ParseCIDR and IP.Equal are logically new in this CL
but accidentally snuck into an earlier CL about unused
labels that was in the same client.

In crypto/tls, drop laddr from Dial to match net.

R=golang-dev, dsymonds, adg, rh
CC=golang-dev
https://golang.org/cl/4244055