Added _bytes_seek and _bytes_tell XDR methods

On error xtc_seek now returns the system errno value, which is printed as
is. This is more informative and fixes an IndexError looking up the error
message, that occurred when xtc_seek failed and returned exdrNR.

Added testing for _bytes_seek and _bytes_tell, also beyond 4GB filesize
limits.

package/CHANGELOG

@@ -26,6 +26,7 @@ API Changes
 
 Enhancement
 
+  * XDR file seeking errors now report system errno. (PR #678)
   * Offsets reading for xtc/trr files has been sped up. (Issue #441)
   * select_atoms now implicitly ORs multiple values after a keyword for
     many types of selections (Issue #345)

package/MDAnalysis/lib/formats/src/xdrfile.c

@@ -41,6 +41,7 @@
 #include <string.h>
 #include <math.h>
 #include <limits.h>
+#include <errno.h>
 
 /* get fixed-width types if we are using ANSI C99 */
 #ifdef HAVE_STDINT_H
@@ -2607,7 +2608,13 @@ xdrstdio_getpos (XDR *xdrs)
 static int
 xdrstdio_setpos (XDR *xdrs, off_t pos, int whence)
 {
-	return fseeko((FILE *) xdrs->x_private, pos, whence) < 0 ? exdrNR : exdrOK;
+    /* A reason for failure can be filesystem limits on allocation units,
+     * before the actual off_t overflow (ext3, with a 4K clustersize,
+     * has a 16TB limit).*/
+    /* We return errno relying on the fact that it is never set to 0 on
+     * success, which means that if an error occurrs it'll never be the same
+     * as exdrOK, and xdr_seek won't be confused.*/
+	return fseeko((FILE *) xdrs->x_private, pos, whence) < 0 ? errno : exdrOK;
 }
 
 
@@ -2621,7 +2628,7 @@ int xdr_seek(XDRFILE *xd, int64_t pos, int whence)
 /* Seeks to position in file */
 {
     int result;
-    if ((result = xdrstdio_setpos(xd->xdr, (off_t) pos, whence)) != exdrOK)
+    if ((result = xdrstdio_setpos(xd->xdr, (off_t) pos, whence)) != 0)
         return result;
 
     return exdrOK;