isisd: Attached-bit in LSP header

isisd: Attached-bit in LSP header

Set/reset attached-bit in LSP header:
    This patch provides support for set/reset attached_bit in the LSP header.
    In IS-IS networks, routing inter-area traffic from L1 areas is
    accomplished by sending the traffic to the nearest L1/L2 router.
    A L1/L2 router identifies itself by setting an attach-bit (ATT-bit) in its (LSP).

    The ATT-bit in LSP can be changed using the set-attached-bit or
    no-set-attached-bit commands (similar to ‘set-overload-bit’ and
    'no set-overload-bit’) using telnet terminal in router configuration mode.

    Steps:
    enable
    configure terminal
    router isis <Routing area>
    set-attached-bit

    V2: Removed looping through area list as this well set the bit for all
        areas in the list. This implementation now looks exactly like the
        current overload bit implementation.

    Signed-off-by: Amritha Nambiar <amritha.nambiar@intel.com>