isisd: Add support for RFC6119 (IPv6 TE in IS-IS)

isisd: Add support for RFC6119 (IPv6 TE in IS-IS)

 - Add advertisement of Global IPv6 address in IIH pdu
 - Add new CLI to set IPv6 Router ID
 - Add advertisement of IPv6 Router ID
 - Correctly advertise IPv6 local and neighbor addresses in Extended IS and MT
   Reachability TLVs
 - Correct output of Neighbor IPv6 address in 'show isis database detail'
 - Manage IPv6 addresses advertisement and corresponiding Adjacency SID when
   IS-IS is not using Multi-Topology by introducing a new ISIS_MT_DISABLE
   value for mtid (== 4096 i.e. first reserved flag set to 1)

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>