changes is:
- http2 package replaces spdy. New interactive HTTP2 debugger, h2i.
- New context/ctxhttp for context-aware HTTP request handlers.
- New xsrftoken package for generating and checking XSRF tokens.
- Improved HTML5-capable HTML parser.
Go packages now define a set of files to buildlink in their buildlink3.mk.
go-packages.mk no longer looks in ${PREFIX}/gopkg during the build. This
should also fix the spurious issues with rebuilds of .a files during bulk
builds of Go packages.