Log Message:
Use lang/go111, not lang/go, for package builds.

This moves builds of packages using Go off the unversioned lang/go package
and onto Go 1.11 or Go 1.9 (on NetBSD 6) by default.

There is a new, user-settable variable GO_VERSION_DEFAULT.

NOTE: not all Go packages completely implement this yet. For example,
net/syncthing does its own thing. This will be fixed.