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.

/!\ BROKEN BUILD /!\

This commit moves the library code to the library directory.  This
causes the firmware to not compile.  A few changes were made to reduce
the nubmer of errors, but some errors just aren't going to go away until
I go ahead with the planned changes.

There are now a few extra lines in the Makefile that make the PD Buddy
firmware library's makefile get included, and the variables it defines
get used.  There's no code to compile or include yet though.