polypath: Reorganise to break up ifaddr handling

polypath: Reorganise to break up ifaddr handling

We are going to want to do some of this processing in a different
process, and hence in a different order.  So split
polypath_process_monitor_lineinto three, producing two new functions
polypath_make_socket and polypath_record_ifaddr.

We change the error handling so it can be more context-sensitive: the
BAD macros use contextual `bad' and `badctx' values.  These are
function pointers.

dump_ppml becomes dump_pria because it records interface address
recording.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>