[release-branch.go1.5] net: restore LookupPort for integer strings

[release-branch.go1.5] net: restore LookupPort for integer strings

This worked in Go 1.4 but was lost in the "pure Go" lookup
routines substituted late in the Go 1.5 cycle.

Fixes #12263.

Change-Id: I77ec9d97cd8e67ace99d6ac965e5bc16c151ba83
Reviewed-on: https://go-review.googlesource.com/13915
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/14243
Reviewed-by: Ian Lance Taylor <iant@golang.org>