Recent advances in pkg_add

[Marc Espie <espie_(_at_)_nerim_(_dot_)_net>]

Current has seen quite a few changes, that you may wish to test.

- pkg_add -u now proceeds with the actual update.
- there's an interactive mode with pkg_add -i that actually asks questions.
- I finally implemented PKG_CACHE: you can set the env variable to point 
to a directory, and any package downloaded through ftp/http/scp will be
copied there, in addition to being installed.

And there's much more, but that's probably the most user-visible parts.