net: clean up cgo

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/9020
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/error_test.go

@@ -84,7 +84,7 @@ second:
 		return nil
 	}
 	switch err := nestedErr.(type) {
-	case *AddrError, *DNSError, InvalidAddrError, *ParseError, UnknownNetworkError, *timeoutError:
+	case *AddrError, addrinfoErrno, *DNSError, InvalidAddrError, *ParseError, *timeoutError, UnknownNetworkError:
 		return nil
 	case *DNSConfigError:
 		nestedErr = err.Err