patch 8.0.0448: some macros are in lower case

Problem:    Some macros are in lower case, which can be confusing.
Solution:   Make a few lower case macros upper case.
diff --git a/src/undo.c b/src/undo.c
index 5b95379..833deca 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -2784,7 +2784,7 @@
 
     curhead->uh_entry = newlist;
     curhead->uh_flags = new_flags;
-    if ((old_flags & UH_EMPTYBUF) && bufempty())
+    if ((old_flags & UH_EMPTYBUF) && BUFEMPTY())
 	curbuf->b_ml.ml_flags |= ML_EMPTY;
     if (old_flags & UH_CHANGED)
 	changed();
@@ -3175,14 +3175,14 @@
 	if (STRCMP(ml_get_buf(curbuf, lnum, FALSE),
 						uep->ue_array[lnum - 1]) != 0)
 	{
-	    clearpos(&(uhp->uh_cursor));
+	    CLEAR_POS(&(uhp->uh_cursor));
 	    uhp->uh_cursor.lnum = lnum;
 	    return;
 	}
     if (curbuf->b_ml.ml_line_count != uep->ue_size)
     {
 	/* lines added or deleted at the end, put the cursor there */
-	clearpos(&(uhp->uh_cursor));
+	CLEAR_POS(&(uhp->uh_cursor));
 	uhp->uh_cursor.lnum = lnum;
     }
 }