Previous approach, reverted in commit c7cf2ca0, would break unnamed
sockets - since it assumed there would always be an interim step were
one would first name an unnamed socket, before "listen" / "connect".

Instead of using the same approach as the "tcp:" scheme in Redox's
netstack, which requires an extra mapping of temporary unnamed sockets,
handle the different use cases in dup(). Thus, if a "buf" is provided
that's not a "connect" or "listen", it must be a new path for turning
the unnamed socket into a named socket and should be handled as such.