Enhanced tracebacks, with local variables

Add a new component, phlsys_traceback which has support for printing
enhanced tracebacks with more information for debugging purposes.

Test Plan:
$ ./precommit.sh

Try installing the exception handler in various existing apps to see
that the output is as expected.  Some of the traces are a little
strange, possibly owing to outputting the contents of local variables.
Nothing appears to be actually wrong however.
Arcyd: install enhanced tracebacks

Install the new enchanced tracebacks to sys.excepthook and when mailing
exceptions around.

Be conservative and send two mails, the first with the regular message
and the second with the enhanced message; as the mechanism is currently
considered to be experimental.

Test Plan:
$ ./precommit.sh
WIP: abdweb