Make datapath interface library more flexible.

Make datapath interface library more flexible.

A single netlink socket can address any kernel datapath, but the datapath
interface library was artificially restricting a dpif to address a single
datapath that was selected at dpif creation time.  This commit removes
that restriction.

This is useful for vswitchd, which can create and destroy any number of
datapaths and add and remove ports on them.  It's more convenient to use
a single dpif for that instead of many.