Standard fields not accepted in logging format strings

[freddrake]

The process, processName, thread, and threadName fields are not currently considered valid by ZConfig.

These may, individually, be None or a value of the appropriate type, depending on the values of globals in the logging module.

This is at least tangentially related to #53, but may require special consideration given the relation to the logging.logThreads and logging.logMultiprocessing settings.

[freddrake]

funcName is another that's been omitted; it wasn't around when ZConfig was written, but has been since Python 2.6.

That one should be very safe to add a string sample value; it appears to always be present as a string, even though it might be an uninformative '(unknown function)' at times.

[freddrake]

All of these fields are actually present when configuring a formatter using the defined sections, so validation of the fields works as expected, with the default settings of logging.logMultiprocessing and logging.logThreads.

The old validation function for format strings does not support these, though the updates to reflect proper types for other fields affect it as well.