isisd: Fix Attach-bit processing

The purpose of the Attach-bit is to accomplish inter-area routing.  In other
venders, the Attached-bit is automatically set when a router is configured
as a L1|L2 router and has two adjacencies.  When a L1 router receives a LSP
with the Attached-bit set it is supposed to create a default route pointing
toward the neighbor to provide a default path out of the L1 area.

ISIS implementation has been fixed to support the above definition:
Setting the Attach-bit is now the default behavior and we allow the user to
turn it off.

We will only set the Default Attach-bit when creating a L1 LSP, if we are
a L1|L2 router and have a L2 adjacency up.

When a L1 router receives a LSP with the Attach-bit set, we will create a
default route pointing to the L1|L2 router as the nexthop.

The default route will be removed if the LSP is received with the Attach-bit
cleared.

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

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