bluez: track adapter address in NMBluezDevice

We'll need it for bluez5 DUN support.

[lkundrak@v3.sk: Turn the addresses to strings from guint8[ETH_ALEN], as that
is what rest of NetworkManager uses for MAC addresses and what Bluez utility
functions expect as well.]
bluez: re-add DUN support for Bluez5

This adds service discovery via SDP and RFCOMM tty management to
NetworkManager, as it was dropped from Bluez.

Based on work by Dan Williams <dcbw@redhat.com>.
The SDP discovery is based on code from Bluez project.