parse_prefix in parse.c got changed but the declaration in bgpctl.c

parse_prefix in parse.c got changed but the declaration in bgpctl.c
wasn't updated, so we would crash when doing `bgpctl net bulk` commands.

Fix by moving parse_prefix into a header, since we use it in more than
one file.

crash found by henning@
underlying problem found by blambert@

OK sthen@ deraadt@ claudio@ henning@