logind: fix PropertiesChanged parser

logind: fix PropertiesChanged parser

The current parser directly reads a BOOLEAN on the PropertiesChanged
signal for 'Active' properties. However, all property-values are packed in
a VARIANT, otherwise, we wouldn't know the type. Fix the parser to recurse
into the variant before reading the boolean.

To avoid such bugs in the future, we extract the 'Active' parser into a
helper function parse_active(), which is then shared between the
PropertiesChanged and Get handlers.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Tested-by: nerdopolis <bluescreen_avenger@verizon.net>