dpif: Replace dpif_id() by dpif_name().

dpif: Replace dpif_id() by dpif_name().

dpif_id() is often used in error messages, e.g. "dp%u: screwed up".  But
soon we will be generalizing the concept of a datapath, so it is better
to have a function that returns a full name, e.g. "%s: screwed up".
Accordingly, this commit replaces dpif_id() by a new function dpif_name()
that does so.