This patch slighly modifies ethertap in 2 ways:
1. it uses dev_alloc_name() to get the net_device name and no longer
   requires the user to know and manually set the variable unit.
   (but the code still supports unit when provided)
2. add NETLINK_TAPEND in netlink.h to 'officialize' the range of 
   ethertap device and do a sanity check