mrtg-ubnt-probe.py: The dotted notation must be instantiated with a dict object, so
make sure that is always the case by shifting around a couple of things. Also make sure
to accomodate user provided keys that start with a list item.
mrtg-ubnt-probe.py: Content-type check needs an exception for AirOS sta.cgi
mrtg-ubnt-probe.py: Partially revert f7b32692 so as to avoid dealing with
special characters in source, e.g. status-new contains an operator which
breaks eval().