ospf6d: decimal area format in interface command

ospf6d: decimal area format in interface command

The ospf6 "interface <blah> area <x>" command only allows the area to
be specified in the ipv4 address format, whereas the show run command
always shows it in the format in which the area was created. This causes
the frr-reload script to be unable to remove ospfv3 interfaces when the
area was created in decimal format. The solution is to allow both formats
to be configured as they can be for other area commands.

Signed-off-by: Duncan Eastoe <duncan.eastoe@att.com>