dbus: clean up new D-Bus interface getters and setters

dbus: clean up new D-Bus interface getters and setters

A number of fixes/improvements here:

1) Remove casting of getter/setter function types which allows
us to change the prototypes in the future and not have hard-to-find
runtime segfaults

2) Instead of having the getters create a fake reply message which
then gets its arguments copied into the real reply message, and is
then disposed, just pass message iters around and have them add
their arguments to the message itself

3) For setters, just pass in the message iter positioned at the
start of the argument list, instead of each setter having to skip
over the standard interface+property name

4) Convert error handling to use DBusError and return the error
back down through the call stacks to the function that will
actually send the error back to the caller, instead of having a
fake DBusMessage of type DBUS_MESSAGE_TYPE_ERROR that then
needs to have the error extracted from it.

But most of all, this fixes various segfaults (like rh #725517
and #678625) which were caused by some functions deep down in the
getter callpaths wanting a source DBusMessage* when the getters were
used for two things: signals (which don't have a source DBusMessage)
and methods (which will have a source DBusMessage that's being
replied to).  This duality made the code fragile when handling
errors like invalid IEs over the air.

Signed-off-by: Dan Williams <dcbw@redhat.com>