Enhancements to pdbtrack.

Enhancements to pdbtrack.

pdbtrack now handles correctly the case where the stacktrace
information is on the second line.

All python buffers not opened by the user and used for tracking are
closed automatically when tracking process finishes.

Simplified code for keeping the tracked buffer.

Removed vars:
 + python-pdbtrack-tracking-buffers

Removed functions:
 + python-pdbtrack-get-or-add-tracking-buffers

New vars:
 + python-pdbtrack-tracked-buffer
 + python-pdbtrack-buffers-to-kill

New functions:
 + python-pdbtrack-set-tracked-buffer