See #1345.
https://github.com/jstasiak/python-zeroconf looks a lot better than the pybonjour code, only downside is that this does not support win32..
Works on macos, so we can drop pybonjour there.
The zeroconf listener had already been added in r15354.
r19262 adds the publisher.
Some big problems remain:
etc..
So this backend is not enabled by default, use XPRA_PREFER_ZEROCONF=1
to enable it.
Note: zeroconf 0.20 is no longer compatible with python2, see ticket:1883#comment:5
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1644