Allow Twisted to accept AF_UNIX sockets from systemd
|Reported by:||rwall||Owned by:|
Allow systemd strports to include a UNIX domain parameter for
inherited UNIX sockets.
Also remember to update the systemd deployment howto which currently
lists this as a known limitation.
Having followed through the code, this may just work after #5573,
where PosixReactorBase.adoptStreamPort is updated to handle AF_UNIX