Enable directory service over tethering interfaces

[usefulsoftwaredev]

See issue https://github.com/occasionaldevshare/DroidUPnP/issues/1

[usefulsoftwaredev]

The main idea of the patch is: if trying to get addresses of usb0 and wlan0 failed, then:

• try to check whether tethering is enabled via wifi If yes:
• try to get ip address of gateway on the tethering interface (should be ip of the device owning the interface)
• if it failed, use 192.168.43.1 (which is the default for tethering interface over wifi, see aosp code).

This patch could be "expanded" (there is a default ip also for tethering over usb, 192.168.42.1 or 192.168.44.1, I do not remember). In situations when there are tethering interfaces (see code of pull request), and wifi tethering is disabled (see also code of pull request), then default ip address when tethering is enabled via usb could be used. Maybe special care will be needed for cases when tethering is enabled over bluetooth...