Expose Frame Type and Stop Reason in python Unwinder API

Hello,

Recently I wrote a kernel trapframe unwinder using the python API, and
I was able to successfully extract the registers from the trapframe.
However, sometimes gdb would throw the "previous frame inner to this
frame" error on me.  This is because unwinding a trapframe sometimes
results in unwinding to a different stack.

The way this is fixed in the internal unwinder API is that there are
special frame types such as SIGTRAMP_FRAME which don't get checked for
that error.

Additionally, some trapframes unwind into userspace.  In those cases
I'd like to be able to stop the backtrace by setting a stop reason.
More broadly this would be useful for any case where a user wants to
stop the stacktrace at a certain point for whatever their use case is.

I'm interested in moving forward and writing a patch for this.

One implementation detail I noticed that complicates this
implementation is that all python unwinders are internally modeled as
a single C unwinder in py-unwind.c:974.  I believe that in order to
support a frame type feature, each unwinder will need to be modeled as
an independent C unwinder.

I am proposing this here to get community feedback on whether this
feature should be supported.

Thanks,
Andrew Wock