bgpd, vtysh: Add support for route tags

[Forward ported by Cumulus]

Documentation
-------------
All ipv4 and ipv6 static route commands now have a "tag" option
which allows the user to set a tag between 1 and 65535.

quagga(config)# ip route 1.1.1.1/32 10.1.1.1 tag ?
  <1-65535>  Tag value
quagga(config)# ip route 1.1.1.1/32 10.1.1.1 tag 40
quagga(config)#

quagga# show ip route 1.1.1.1/32
Routing entry for 1.1.1.1/32
  Known via "static", distance 1, metric 0, tag 40, best
  * 10.1.1.1, via swp1

quagga#

The route-map parser supports matching on tags and setting tags
!
route-map MATCH_TAG_18 permit 10
 match tag 18
!

!
route-map SET_TAG_22 permit 10
 set tag 22
!

BGP and OSPF support:
- matching on tags when redistribing routes from the RIB into BGP/OSPF.
- setting tags when redistribing routes from the RIB into BGP/OSPF.

BGP also supports setting a tag via a table-map, when installing BGP
routes into the RIB.

Signed-off-by: Daniel Walton <dwalton@cumulusnetworks.com>
Signed-off-by: Piotr Chytla <pch@packetconsulting.pl>
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>

Edits by: Paul Jakma <paul.jakma@hpe.com - conflicts on re-ordering with the
rmap-event and table-map patches, those will now need to update the tags stuff.
diff --git a/bgpd/bgp_attr.h b/bgpd/bgp_attr.h
index 000ceaa..0bbb912 100644
--- a/bgpd/bgp_attr.h
+++ b/bgpd/bgp_attr.h
@@ -92,6 +92,9 @@
 
   uint16_t			encap_tunneltype;	/* grr */
   struct bgp_attr_encap_subtlv *encap_subtlvs;		/* rfc5512 */
+
+  /* route tag */
+  u_short tag;
 };
 
 /* BGP core attribute structure. */