This CL makes it possible to resolve DNS names on OS X
without offending the Application-Level Firewall.

It also means that cross-compiling from one operating
system to another is no longer possible when using
package net, because cgo needs to be able to sniff around
the local C libraries.  We could special-case this one use
and check in generated files, but it seems more trouble
than it's worth.  Cross compiling is dead anyway.

It is still possible to use either GOARCH=amd64 or GOARCH=386
on typical Linux and OS X x86 systems.

It is also still possible to build GOOS=linux GOARCH=arm on
any system, because arm is for now excluded from this change
(there is no cgo for arm yet).

R=iant, r, mikioh
CC=golang-dev
https://golang.org/cl/4437053