python.el: Pdbtracking improvements

python.el: Pdbtracking improvements

Allow not to kill buffers when pdbtracking session is finished.
Pdbtracking session considered finished judging from the user input.

* lisp/progmodes/python.el (python-pdbtrack-kill-buffers): New
customizable variable.
(python-pdbtrack-set-tracked-buffer): Use it.
(python-pdbtrack-unset-tracked-buffer)
(python-pdbtrack-tracking-finish): New functions.
(python-pdbtrack-continue-command, python-pdbtrack-exit-command):
New customizable variables.
(python-pdbtrack-process-sentinel): New function.  Finish
pdbtracking session when process is killed.
(python-pdbtrack-prev-command-continue): New variable.
(python-pdbtrack-comint-input-filter-function): New function.
Finish pdbtracking session based on commands sent to pdb.
(python-pdbtrack-comint-output-filter-function): Unset/set
tracking buffer if looking at pdb prompt; finish pdbtracking
session if filename of current stack frame starts with
"<" e.g. "<stdin>".
(python-pdbtrack-comint-output-filter-function): Don't override
overlay-arrow-string.
(python-pdbtrack-setup-tracking): New function.
(inferior-python-mode): Use it.
(python-pdbtrack-stacktrace-info-regexp): Default value is changed.
Must also match lines with filename like "<stdin>" and "<string>".
* etc/NEWS: Mention python-pdbtrack-kill-buffers