Disable deletion of manually-entered neighbors

When a neighbor is manually added to the neighbor list via the xip
utility, that neighbor should not be removed from the neighbor list.
These nodes are kept in the neighbor list to keep routing compatible
in other XIA implementations.

To do this, insert_neigh() and new_ha() are given an additional
parameter, which requires a change in the prototype of insert_neigh()
in include/net/xia_hid.h. Every struct hrdw_addr now also has an
8-bit, unsigned flags field for whether the neighbor should be kept.
It is possible for this flag to take on the following bit values:

  * NWP_KEEP_ENTRY: if neighbor should not be deleted from the
    neighbor list at any time.
  * NWP_CLOCK_MIN: if any clock value is larger than the current
    remote clock.

Therefore, update_ha_sc() and find_update_neigh_sc() need to be aware
of the meanings of these values. clean_neigh_list() also should not
discard any neighbor who has a NWP_KEEP_ENTRY flag.