Drop the platform-specific floor (0.9.12 darwin / 0.9.14 others) in
favour of a single 1.2.0 minimum. This server now relies on features
only present in recent uBridge builds: the AF_UNIX control channel
(-U / SO_PEERCRED), the marker (mark) filter, and the brctl-backed
builtin Ethernet Switch. Removes the now-unused sys import left behind
by the dropped darwin branch (target is Linux-only).
b8ac76f047