zebra: Add ability to support tags -> realms in linux

zebra: Add ability to support tags -> realms in linux

Linux has the ability to support a concept of 'realms'.
This concept allows you to mark routes with a realm id
value of 1-255.  If you have marked the realm
of a route then you can use the tc program to
apply policy to the routes.

This commit adds the ability of FRR to interpret
a tag from (1-255) as a realm when installing into
the kernel.  Please note that at this point in time
there is no way to set policy from within FRR.  This
must be done outside of it.

The normal methodology for setting tags is valid here
via a route-map.

Finally this is only applied if the --enable-realms configure
option is applied.

Signed-off-by: Kaloyan Kovachev <kkovachev@varna.net>