To do this, a couple of the events had to have their definitions moved
to lib/include/pdb_pe.h.  This isn't a problem in and of itself, but if
more internal-use events are added to policy_engine.h there could be
trouble.  To mitigate this, I added a comment as a reminder to make sure
any new events don't overlap the public ones defined elsewhere.

Their names were a bit too greedy for the global namespace, so I
changed them to start with fusb_tcc_.

Not the static globals yet, just the public ones.  I suppose this change
isn't technically necessary since the PD Buddy Sink is a single-port
device, but I want its firmware to be a shining example of how to use
the library.

Now the user-visible FUSB302B functions all take a struct
pdb_fusb_config * as a parameter.  The static functions don't use it
yet, but at least the public API of this code is in its finished state.

It was a hidden static variable of the policy engine, which would've
been a problem in a multi-port device.  Now it won't be anymore.

Apparently one of the threads was using too much stack space??  This
will require investigtion later, but for now increasing their stack
space fixed some weird problems I was having with this patch.

The code compiles again, and it seems to work as it did before.  There's
still a lot to do (we still have a mess of globals), but the PE
correctly calls the new DPM functions via the list of callbacks in the
configuration object.

These new headers define structs for configuring the library.  They're
currently unused, and some function prototypes no longer match.

For now, the new <pdb.h> file only has a function to start the library,
based on the old start_pd function from main.c.  It'll eventually be all
a user needs to include, but there's a lot more refactoring to do.