netdev: Avoid some system calls in the common case in netdev_open().

netdev: Avoid some system calls in the common case in netdev_open().

The new secchan opens one netdev per OpenFlow port.  We should be able to
handle this in the common case without one file descriptor per netdev
(because most netdev operations can be performed using a single AF_INET
socket).  This change starts along that path by moving the operations
that are required only to receive netdev packets out of the common path.