py3: make get_current_authuser handle missing tg context consistently and explicitly

tg context handling ends up using
tg.support.registry.StackedObjectProxy._current_obj for attribute access ...
which if no context has been pushed will end up in:
raise TypeError(
'No object (name: %s) has been registered for this '
'thread' % self.____name__)

utils2.get_current_authuser used code like:
if hasattr(tg.tmpl_context, 'authuser'):

Python 2 hasattr will call __getattr__ and return False if it throws any
exception. (It would thus catch the TypeError and silently fall through to use
the default user None.) This hasattr behavior is confusing and hard to use
correctly. Here, it was used incorrectly. It has been common practice to work
around by using something like:
getattr(x, y, None) is not None

Python 3 hasattr fixed this flaw and only catches AttributeError. The TypeError
would thus (rightfully) be propagated. That is a change that must be handled
when introducing py3 support.

The get_current_authuser code could more clearly and simple and py3-compatible
be written as:
return getattr(tmpl_context, 'authuser', None)
- but then we also have to handle the TypeError explicitly ... which we are
happy to do.