Switch prefix_adjout_get and new prefix_adjout_first to use a pt_entry

Switch prefix_adjout_get and new prefix_adjout_first to use a pt_entry
as argument instead of the bgpd_addr + prefixlen.

Do the same with prefix_adjout_update but leave prefix_adjout_lookup
and prefix_adjout_match since those are used by bgpctl code that does
not use pt_entry structs.

With this most of the update code no longer needs struct bgpd_addr and
pt_getaddr().
OK tb@