More gmxfio simplification

More gmxfio simplification

- Remove gmx_fio_setdebug() and all implementation related to it;
  getting the source code location printed to stderr is not really
  useful for errors that typically originate from the file system.
- Move gmx_fio_setprecision() to gmxfio-xdr, since it logically belongs
  there.
- Remove bOpen, as the only case where any code could observe
  bOpen==FALSE requires a use-after-free, and possibly also an ugly
  race condition on top of that.
- Reduce use of iFTP member within gmxfio, where the purpose is to check
  whether the fio->xdr member is not NULL.
- Remove checks from gmx_fio_open() that overlapped with those in
  gmx_ffopen().  There are some corner cases that now work differently:
  gmxfio no longer bypasses the check for .Z/.gz files for xdr files,
  but lets things through to gmx_ffopen(), and with GMX_FAHCORE there is
  no longer any kind of existence check for xdr files (it is
  questionable whether the previous behavior was intentionally different
  for xdr and non-xdr files, anyways).
- Remove dead code: fio->fp is always non-NULL if fio->xdr is.

Change-Id: Icd1588bfbb510543a55f57fb86e0e1e2a1fce236