vxlan: Allow setting destination to unicast address.

This patch allows setting VXLAN destination to unicast address.
It allows that VXLAN can be used as peer-to-peer tunnel without
multicast.

v4: generalize struct vxlan_dev, "gaddr" is replaced with vxlan_rdst.
    "GROUP" attribute is replaced with "REMOTE".
    they are based by David Stevens's comments.

v3: move a new attribute REMOTE into the last of an enum list
    based by Stephen Hemminger's comments.

v2: use a new attribute REMOTE instead of GROUP based by
    Cong Wang's comments.

Signed-off-by: Atzm Watanabe <atzm@stratosphere.co.jp>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>