Don't call malloc() and free() while in the debugger and unwinding

to get a stacktrace. This does not work even with M_NOWAIT when we
have WITNESS and is generally a bad idea (pointed out by bde@). We
allocate an 8K heap for use by the unwinder when ddb is active. A
stack trace roughly takes up half of that in any case, so we have
some room for complex unwind situations. We don't want to waste too
much space though. Due to the nature of unwinding, we don't worry
too much about fragmentation or performance of unwinding while in
the debugger. For now we have our own heap management, but we may
be able to leverage from existing code at some later time.

While here:
o  Make sure we actually free the unwind environment after unwinding.
   This fixes a memory leak.
o  Replace Doug's license with mine in unwind.c and unwind.h. Both
   files don't have much, if any, of Doug's code left since the EPC
   syscall overhaul and the import of the unwinder.
o  Remove dead code.
o  Replace M_NOWAIT with M_WAITOK for all remaining malloc() calls.

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