Add experimental bridging program for providing D-Bus access over TCP/IP

Add experimental bridging program for providing D-Bus access over TCP/IP

The idea is that this

 # ./udisks-tcp-bridge -p 1000

will listen on port 1000 of all network interfaces. Another host can
then access the udisks daemon via D-Bus' TCP/IP transport. See
gnome-disk-utility master for a an experimental --address option that
uses this.

Since Palimpsest use D-Bus 100% to obtain information and export
operations, everything just works. See

 http://people.freedesktop.org/~david/gdu-palimpsest-remote-connection.png

for an example.

This feature needs a lot of thought and some work - for example,
authentication and encryption is needed. And we need to figure out how
this should work with polkit as well - the only reason it works now is
that the bridging process runs as uid 0. And then there's firewalls to
consider and pierce through. All this is not insurmountable, it's just
work.

For now udisks-tcp-bridge is not installed via 'make install' - it is
considered an experimental feature for now - and will probably remain
so for some time.