pkg_add API

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

Heads up, I've started committing basic API documentation to the pkg_add
tools, e.g., all the stuff I've been using to writing extra scripts like
update-plist or lib-depends-check.

Note well: the documentation isn't finished yet. And the API is not frozen
yet. You can use what's documented to write scripts (and maybe a bit more),
but don't be disappointed if I change stuff, and your script no longer
runs.

The API is slowly getting mature, I expect it to be more or less stable
in ~6 months...