selftest/gdb_backtrace: print python traceback if available

selftest/gdb_backtrace: print python traceback if available

If a program happens to have 'PyList_New' defined but is not a python
script, gdb will print

> Undefined command: "py-bt".  Try "help".

and probably stop. This happens after the C backtraces have been
printed, so nothing is lost.

The traceback is printed twice -- once in conventional Python style
for clarity, and once with extra "full" information.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>