Consolidation with CDI-PIO (develop)

Generally, this is !11 (merged) rebased onto the develop branch.

This is the first develop-based version accepted by the Buildbot of ICON: https://buildbot.dkrz.de/grid?branch=m300488/cdi-pio-2-integration

src/file.c

@@ -380,8 +380,17 @@ fileSetBufferType(int fileID, int type)
 int
 fileFlush(int fileID)
 {
+  int retval = 0;
   bfile_t *fileptr = file_to_pointer(fileID);
-  return fileptr ? fflush(fileptr->fp) : 0;
+  if (fileptr)
+    {
+      FILE *fp = fileptr->fp;
+      retval = fflush(fp);
+      if (retval == 0) retval = fsync(fileno(fp));
+      if (retval != 0) retval = errno;
+    }
+
+  return retval;
 }
 
 void