Disable deletion of manual 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.