[interface configuration] Try to avoid losing address info after shutdown.

[interface configuration] Try to avoid losing address info after shutdown.
2006-05-21 Andrew J. Schorr <ajschorr@alumni.princeton.edu>

	* if.h: (struct connected) Document the meaning of the
	  ZEBRA_IFC_REAL and ZEBRA_IFC_CONFIGURED flags.
	* connected.c: (connected_withdraw) Do not delete the connected
	  address if the ZEBRA_IFC_CONFIGURED flag is set.
	  (connected_add_ipv4,connected_add_ipv6) Before calling
	  connected_withdraw, unset the ZEBRA_IFC_CONFIGURED flag
	  on the superseded connected structure.