patch 8.0.0451: some macros are in lower case

Problem:    Some macros are in lower case.
Solution:   Make a few more macros upper case. Avoid lower case macros use an
            argument twice.
diff --git a/src/mbyte.c b/src/mbyte.c
index 11dc0fb..d9bca6b 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -4047,7 +4047,7 @@
     char_u *p)
 {
     if (p > line)
-	mb_ptr_back(line, p);
+	MB_PTR_BACK(line, p);
     return p;
 }
 
@@ -5708,7 +5708,6 @@
 
 
 #ifdef USE_X11R6_XIM
-static void xim_instantiate_cb(Display *display, XPointer client_data, XPointer	call_data);
 static void xim_destroy_cb(XIM im, XPointer client_data, XPointer call_data);
 
     static void