yang: create interface reference type

yang: create interface reference type

Instead of copy-pasting a 16 character string type for use as an
interface reference, create a new typedef that leafref's the name node
of an interface. This way the constraints change with the constraints on
an interface name itself, and it's self documenting.

Incidentally ripd and ripngd forgot the 16 character constraint in their
offset-list configs and IS-IS forgot it entirely, so this also fixes
minor bugs.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>