net: avoid string operation and make valid domain names explicit

Having a trailing dot in the string doesn't really simplify
the checking loop in isDomainName. Avoid this unnecessary allocation.
Also make the valid domain names more explicit by adding some more
test cases.

benchmark            old ns/op    new ns/op    delta
BenchmarkDNSNames       2420.0        983.0  -59.38%

benchmark           old allocs   new allocs    delta
BenchmarkDNSNames           12            0  -100.00%

benchmark            old bytes    new bytes    delta
BenchmarkDNSNames          336            0  -100.00%

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12662043

Committer: Brad Fitzpatrick <bradfitz@golang.org>