XIP: disable deletion of manually-entered neighbors

When a neighbor is manually added to the neighbor list via the xip
utility, that neighbor should not participate in NWP and should not
be removed from the neighbor list. These nodes are kept in the
neighbor list to keep routing compatible in other XIA implementations,
but they should not be sent in neighbor lists or monitored.

To do this, insert_neigh() and new_ha() are given an additional
parameter permanent. If this parameter is true, then the node has been
manually inserted. Every struct hrdw_addr now has a bool permanent to
reflect this as well.

clean_neigh_list() also should therefore not discard any neighbor who
is considered permanent.