Allow selective auto-indentation of multiline log messages.

Adds command line option ``--log-auto-indent``, config option
``log_auto_indent`` and support for per-entry configuration of
indentation behavior on calls to ``logging.log()``.

Alters the default for auto-indention from ``on`` to ``off``. This
restores the older behavior that existed prior to v4.6.0. This
reversion to earlier behavior was done because it is better to
activate new features that may lead to broken tests explicitly
rather than implicitly.