netdev: Allow explicit creation of netdev objects

This change adds netdev_create() and netdev_destroy() functions to allow
the creation of network devices through the netdev library.  Previously,
network devices had to already exist or be created on demand through
netdev_open().  This caused problems such as not being able to specify
TAP devices as ports in ovs-vswitchd, which this patch fixes.

This also lays the groundwork for adding GRE and VDE support.