lib: Add a basic mlag.h file

lib: Add a basic mlag.h file

This is the start of a series of commits that will allow FRR to
be integrated into mlag.

Zebra and Pim will both need mlag state for the router.  As such we will
need to provide a abstract about this state through the zapi.

This is the start of the common header that both Pim and Zebra will
be using.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>