server: Correct handling of interface names in tun interface.

Now that interface names can be changed, we need to let the tunnel
driver know of changes so that it can produce the right messages when
things go weird.  It's better to let the peer edifice handle interface
names, so

  * let `create' return the interface name rather than have p_ifname ask
    the tunnel explicitly, and

  * replace the `ifname' query with an optional `setifname'
    notification, which is currently used only by the SLIP driver to
    maintain the correct name for its persistent interfaces.