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>
diff --git a/isisd/isisd.h b/isisd/isisd.h
index 5db485f..838a08b 100644
--- a/isisd/isisd.h
+++ b/isisd/isisd.h
@@ -116,6 +116,8 @@
   char is_type;			/* level-1 level-1-2 or level-2-only */
   /* are we overloaded? */
   char overload_bit;
+  /* L1/L2 router identifier for inter-area traffic */
+  char attached_bit;
   u_int16_t lsp_refresh[ISIS_LEVELS];
   /* minimum time allowed before lsp retransmission */
   u_int16_t lsp_gen_interval[ISIS_LEVELS];