time to import pkg?

[Nikolai Lifanov]

I propose importing ports-mgmt/pkg into base for release-pkg branch.

The reasons for it being separate are addressed by packaging base,
which will ease updates to individual utilities like /usr/sbin/pkg.

This should Just Work with privatessl and packaging /usr/sbin/pkg
itself. This will also avoid the precedent of not having the tool
for updating the base system in the base system.

- Nikolai Lifanov