MAJOR: listener: support inheriting a listening fd from the parent

MAJOR: listener: support inheriting a listening fd from the parent

Using the address syntax "fd@<num>", a listener may inherit a file
descriptor that the caller process has already bound and passed as
this number. The fd's socket family is detected using getsockname(),
and the usual initialization is performed through the existing code
for that family, but the socket creation is skipped.

Whether the parent has performed the listen() call or not is not
important as this is detected.

For UNIX sockets, we immediately clear the path after preparing a
socket so that we never remove it in case an abort would happen due
to a late error during startup.