This change refactors the Unix socket / pipe backend to have a ring buffer
containing segments, where each segment has an optional leading ancillary
buffer containing control messages followed by a normal data buffer.

The SCM_RIGHTS control message has been implemented which transfers file
descriptors to the receiving process. File descriptors are reference counted
and cycles are prevented using the following restrictions:

1) Unix sockets cannot be sent on themselves (on either end).
2) Unix sockets themselves being sent cannot be sent on.
3) Unix sockets cannot send a Unix socket being sent on.

This is a compatible ABI change.