Support per-repository sets

[sputnick@…]

Currently, at least judging from the docs, only one location for specifying custom sets is supported (/etc/portage/sets). Since often, sets are repository-specific, it should be possible to define and use sets defined in overlays. Portage sorta does this (you can put a sets.conf in the overlay root) already, though it's not fully working yet and still work in progress, afaiu.

Extra bonus points for making the sets configuration compatible to portage.

[ferringb]

Propose something please;

re: sets.conf in the repo, realistically supporting that is going to be a massive pita due to the fact it's portage specific namespace- meaning I'll have to maintaining mappings indefinitely. Need to take that one up w/ genone...