lib: move check_bit into prefix common code

lib: move check_bit into prefix common code

Make one version of check prefix bit, and put it inline
with proper prototype. This gets rid of some macro's and also some
assert() that can never happen on a non-broken compiler.

* bgpd/bgp_table.c
  * CHECK_BIT(): sayonara
  * check_bit(): sayonara
  * SET_LINK(): sayonara
  * set_link(): make use of prefix_bit() instead of check_bit()
  * bgp_node_match(): idem
  * bgp_node_lookup(): idem
  * bgp_node_get(): idem
* lib/prefix.h
  * prefix_bit(): new inline version of check_bit()
* lib/table.c
  * CHECK_BIT(): sayonara
  * check_bit(): sayonara
  * SET_LINK(): sayonara
  * set_link(): make use of prefix_bit() instead of check_bit()
  * route_node_match(): idem
  * route_node_lookup(): idem
  * route_node_get(): idem
* ospf6d/ospf6_lsdb.c
  * CHECK_BIT(): sayonara
  * ospf6_lsdb_lookup_next(): make use of prefix_bit() instead of
    CHECK_BIT()
  * ospf6_lsdb_type_router_head(): idem
  * ospf6_lsdb_type_head(): idem
* ospf6d/ospf6_route.c
  * CHECK_BIT(): sayonara
  * ospf6_route_match_head() make use of prefix_bit() instead of
  * CHECK_BIT()