Add support for overlays

overlays are a way to help users to integrate their own ports tree
with the official ports tree without having to maintain clone of the
official tree and remerge on regular basis.

The ports tree will lookup in the overlays (in the order the are listed in
OVERLAY variable) for the dependencies and the USES. It will use the first
found.

in order to use it the user have to declare his overlays that way in their
make.conf:
OVERLAYS= overlay1 overlay2 overlay3

Reviewed by: manu
Approved by: swills
Differential Revision: https://reviews.freebsd.org/D21468