New ports tree feature

[Nikolay Sturm <sturm_(_at_)_erisiandiscord_(_dot_)_de>]

Hi,

as you might have noticed I committed a new feature to the ports tree
(idea from Marc Espie, done on the Portathon in Venice). Here is a quick
explanation of what it does, so you can actually use it. :) The new
feature is activated by setting 

FETCH_PACKAGES=Yes

in /etc/mk.conf or on the command line of your make invocation. What this
does is pkg_add will look for missing packages in $PKG_PATH, which needs
to be defined in your environment (will always be used by pkg_add) or in
/etc/mk.conf (will currently only be used for this feature).

The good thing is, our packages contain information about most (all?)
libraries programs are linked against. So if package foo-1.3 is missing
and is found on your ftp server but is linked against libc.so.39.3 while
your are still at libc.so.38.1, fear not, the package won't be fetched.

To make it more fun, define

FORCE_UPDATE=Yes

as well in /etc/mk.conf, and you will basically stay up2date quite easily.

Now everyone go and update your package tools to -current (it won't work
reliably otherwise), set these variables and report back in case of any
problem you find.

Nikolay