vxlan: Allow setting destination to unicast address.

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.

v6: change back to the v3 except for using new attribute because
    replacing command-line parameters breaks existing scripts,
    based by Cong Wang's comments.

v5: rebase on the latest.

v4: replace "group" with "remote" 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.
    fix the usage to show explicitly that both "remote" and "group"
    cannot be specified, based by Ben Hutchings's comments.

v2: use a new argument "remote" instead of "group" based by
    Stephen Hemminger's comments.

Signed-off-by: Atzm Watanabe <atzm@stratosphere.co.jp>