netdev-linux: Give tap FD to first opener.

netdev-linux: Give tap FD to first opener.

Tap devices can have two FDs that allow transmit and receive from
different perspectives.  We previously would always share one of
the FDs among all openers.  However, this is confusing to some
users (primarily the DHCP client) which expect tap devices to behave
like any other device.  Now we give the tap FD to the first opener,
which knows that it has opened a tap device, and a normal system FD
to everyone else for consistency.