common/dbus: systemd-bus based murphy-dbus implementation.

common/dbus: systemd-bus based murphy-dbus implementation.

Added an alternative murphy-dbus implementation based on the
low-level D-Bus library (systemd-bus) from systemd. This needs
a patched systemd to build and install systemd-bus as a shared
library.

To get rid of libdbus-dependency it was also necessary to provide
an abstraction for message building and parsing and hence break
backward compatibility. To ease the transitional pain, we provide
currently three version of murphy-dbus: the original one with the
leaky message abstraction, as well as a libdbus- and a systmed-bus-
based one with the new abstraction. Once all the code has been
updated to not use directly libdbus calls for message building and
parsing, the original library will be removed.