datapath: Make datapath-protocol.h portable to non-Linux systems.

datapath: Make datapath-protocol.h portable to non-Linux systems.

datapath-protocol.h is not a very clean interface.  I originally intended
it to be solely a Linux-kernel specific interface.  Over time it became
a general-purpose interface to dpifs.  This is not a good situation,
because clearly the header is still Linux-specific.

In the long run, the correct solution is to separate the generic and
Linux-specific bits.  This is not that patch.  Instead, this patch modifies
datapath-protocol.h enough that it can be used on non-Linux hosts.  In
particular I tested that it works OK with FreeBSD 8.0.