isisd: When adjacencies go up and down add support to modify attached-bit

isisd: When adjacencies go up and down add support to modify attached-bit

When adjacencies change state the attached-bits in LSPs in other areas
on the router may need to be modified.

 1. If a router no longer has a L2 adjacency to another area the
    attached-bit must no longer be sent in the LSP
 2. If a new L2 adjacency comes up in a different area then the
    attached-bit should be sent in the LSP

Signed-off-by: Lynne Morrison <lynne@voltanet.io>