vswitch: Remove restriction on datapath names.

vswitch: Remove restriction on datapath names.

Commit f4b96c92c "vswitch: Disallow bridges named "dpN" or "nl:N"" disabled
naming bridges "dpN" because the vswitchd code made the bad assumption that
the bridge's local port has the same name as the bridge, which was not
true (at the time) for bridges named dpN.  Now that assumption has been
eliminated, so this commit eliminates the restriction too.

This change is also a cleanup in that it eliminates one form of the
vswitch's dependence on specifics of the dpif implementation.