isisd: fix crash on "no router isis" (BZ#536)

The crash is due to threads accessing data that gets destroyed
during the removal of the configuration.

* isis_circuit.c: Destroy adjacencies to stop adjacency expiry thread.
  Stop PSNP threads.
* isisd.c: Change state of circuit back to INIT and reassign the
  circuit structure to isis->init_circ_list rather than destroying
  the circuit data structure. Stop SPF threads. Stop LSP generation
  threads.
* isisd.h: Add pointers to LSP threads into area structure in order to
  stop them in isisd.c
* isis_lsp.c: Store pointer to LSP thread in area structure.
* isis_pdu.c: Stop PDU generation for a circuit with a removed area.
* isis_pfpacket.c: Stop processing received PDUs for a circuit with a
  removed area.
diff --git a/isisd/isisd.h b/isisd/isisd.h
index 2277f27..b17982e 100644
--- a/isisd/isisd.h
+++ b/isisd/isisd.h
@@ -93,6 +93,8 @@
   struct flags flags;
   struct thread *t_tick;	/* LSP walker */
   struct thread *t_remove_aged;
+  struct thread *t_lsp_l1_regenerate;
+  struct thread *t_lsp_l2_regenerate;
   int lsp_regenerate_pending[ISIS_LEVELS];
   struct thread *t_lsp_refresh[ISIS_LEVELS];