dpif: Rename dpif_get_name() to dpif_port_get_name(), update interface.

dpif: Rename dpif_get_name() to dpif_port_get_name(), update interface.

With multiple kinds of datapaths, code should not just use
"dp%u" along with dpif_minor() to print a datapath name, because not all
datapaths can sensibly be named that way.  We want to use a function
with a name like dpif_get_name() to retrieve a datapath name for printing
to the user, in which case the existing dpif_get_name() function would be
confusing.  So rename the existing one to something more explicit.