Now simple Sinks are easier to implement, as not all DPM functions have
to be provided.

All those comments in pdb_dpm.h aren't needed anymore.  They were just
to help me organize my thoughts on how the DPM should work.

All the internal functions and definitions have been moved out of
lib/include/.  Includes are made with the semantically correct brackets
("" or <>) throughout the firmware and library.  That is to say, the
library is essentially done at this point, with only documentation
changes left to be made.

This commit splits the old messages.h file into two, one private
(messages.h) and one public (pdb_msg.h).  This new arrangement keeps the
old model of one message pool for all the PD Buddy firmware library, but
moves the macro for the number of objects it contains into a new file,
pdb_conf.h, placed at the project root.  The pool itself is kept in the
new pdb_msg.h file, since the DPM may need to access it.

The new configuration file also contains the stack sizes for the PD
Buddy threads (some will be moved back to secrecy in the future once the
needed sizes are known, but the PE's stack size should be kept in
pdb_conf.h since user code needs that stack).

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.