isisd: add support for segment routing

isisd: add support for segment routing

This is an implementation of the IS-IS SR draft [1] for FRR.

The following features are supported:
* IPv4 and IPv6 Prefix-SIDs;
* IPv4 and IPv6 Adj-SIDs and LAN-Adj-SIDs;
* Index and absolute labels;
* The no-php and explicit-null Prefix-SID flags;
* Full integration with the Label Manager.

Known limitations:
* No support for Anycast-SIDs;
* No support for the SID/Label Binding TLV (required for LDP interop).
* No support for persistent Adj-SIDs;
* No support for multiple SRGBs.

[1] draft-ietf-isis-segment-routing-extensions-25

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>