Don't install headers depending on GMX_INTERNAL_XDR

There should be no need to install xdrf.h; if some user wants to use the
API, some higher-level API is probably better suitable.  With some
reorganization of the headers, all cases where the header was included
from some other installed header were actually unnecessary:
 - gmx_fio_getxdr() is now in xdrf.h (somewhat ugly, but so are the
   alternatives).
 - xdr_datatype is mainly used for edr reading (also for checkpoint
   precision), but put it into a separate header for now that can be
   installed.  It is actually completely independent of the other stuff
   in xdrf.h.

Change-Id: I74a66388a8c09cbf4eb9b7b44d7cd13be2776e9c