Listener keys (socket address, plus interface when bound) and file
descriptors of listening sockets, needed by a
Proxy to start listening. The transport is fd-type-agnostic: udp carries
UdpSocket fds, the others carry TcpListener fds.
Get a localhost port for a test listener, held by reserve_port:
unique across concurrent processes that all allocate their ports
through reserve_port, within that function’s limits.
Reserve 127.0.0.1:port against the other processes on this machine
that also reserve their ports through this function, until this
process has reserved 256 more ports. Returns false, reserving
nothing, when the port is already in use.