Distinguish between ignored and malformed packets

When there is a peer speaking PTPv1 in the network we want to silently ignore
the packets instead of flooding system log with error messages.  At the same
time we still want to report malformed packets.  For that we reuse standard
error numbers and do more fine-grained error reporting in packet processing
routines.

Signed-off-by: Libor Pechacek <lpechacek@suse.cz>