bgpd, zebra: Support for sticky MACs

bgpd, zebra: Support for sticky MACs

Implement support for sticky (static) MACs. This includes the following:

- Recognize MAC is static (using NUD_NOARP flag) and inform BGP
- Construct MAC mobility extended community for sticky MACs as per
RFC 7432 section 15.2
- Inform to zebra that remote MAC is sticky, where appropriate
- Install sticky MACs into the kernel with the right flag
- Appropriate handling in route selection

Signed-off-by: Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>