This change adds a type addrinfoErrno to represent getaddrinfo,
getnameinfo-specific errors, and uses it in cgo-based lookup functions.

Also retags cgo files for clarification and does minor cleanup.

Change-Id: I6db7130ad7bf35bbd4e8839a97759e1364c43828
Reviewed-on: https://go-review.googlesource.com/9020Reviewed-by: Ian Lance Taylor <iant@golang.org>