whiteknight/pbc_pbc branch needs testing

[Andrew Whitworth]

I have been working on several cleanups for the packfiles subsystem,
especially the public API for it, in the whiteknight/pbc_pbc branch.
Some changes I've made are straight-forward and should have no effects
on most users. Others are a little bit more controversial (see TT
#1324, for example). I would like to get testing on it to make sure
it's moving in a good direction.

This branch will not merge until after 3.6, if all goes well. Priority
is still on testing master until 3.6 is out. If you have a few spare
cycles, getting tests on this branch would be much appreciated.

We're really starting to get this subsystem interface cleaned up. Not
just the way the functions are named and things like that, but we're
actually starting to make some real changes to the way the interface
is used and what kinds of capabilities and semantics get exposed. I've
removed a lot of dead code, and I've started adding some new features
to enable some deprecation issues to start moving forward as well. I
have a few other things I would like to try to tackle before merging,
but I might not want to wait if there are blockers or deprecation
boundaries in the way (TT #2140, TT #2144, TT #2135, etc).

Thanks,

--Andrew Whitworth