OSPFD: Add Experimental Segment Routing support

OSPFD: Add Experimental Segment Routing support

This is an implementation of draft-ietf-ospf-segment-routing-extensions-24
and RFC7684 for Extended Link & Prefix Opaque LSA.
Look to doc/OSPF_SR.rst for implementation details & known limitations.

New files:

 - ospfd/ospf_sr.h: Segment Routing structure definition (SubTLVs + SRDB)
 - ospfd/ospf_sr.c: Main functions for Segment Routing support
 - ospfd/ospf_ext.h: TLVs and SubTLVs definition for RFC7684
 - ospfd/ospf_ext.c: RFC7684 Extended Link / Prefix implementation
 - doc/OSPF-SRr.rst: Documentation

Modified Files:

 - doc/ospfd.texi: Add new Segment Routing CLI command definition
 - lib/command.h: Add new string command for Segment Routing CLI
 - lib/mpls.h: Add default value for SRGB
 - lib/route_types.txt: Add new OSPF Segment Routing route type
 - ospfd/ospf_dump.[c,h]: Add OSPF SR debug
 - ospfd/ospf_memory.[c,h]: Add new Segment Routing memory type
 - ospfd/ospf_opaque.[c,h]: Add ospf_sr_init() starting function
 - ospfd/ospf_ri.c: Add new functions to Set/Get Segment Routing TLVs
Add new ospf_router_info_lsa_upadte() to send Opaque LSA to ospf_sr.c()
 - ospfd/ospf_ri.h: Add new Router Information SR SubTLVs
 - ospfd/ospf_spf.c: Add new scheduler when running SPF to trigger
update of NHLFE
 - ospfd/ospfd.h: Add new thread for Segment Routing scheduler
 - ospfd/subdir.am: Add new files
 - vtysh/Makefile.am: Add new ospf_sr.c file for vtysh
 - zebra/kernel_netlink.c: Add new OSPF_SR route type
 - zebra/rt_netlink.[c,h]: Add new OSPF_SR route type
 - zebra/zebra_mpls.h: Add new OSPF_SR route type

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