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/ex_getln.c b/src/ex_getln.c
index d3fda24..01286ac 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -4296,7 +4296,7 @@
 	    t = p;
 	    had_sep = FALSE;
 	}
-	mb_ptr_adv(p);
+	MB_PTR_ADV(p);
     }
     return t;
 }
@@ -5370,7 +5370,7 @@
 	if (e - 4 > s && STRNICMP(e - 4, ".vim", 4) == 0)
 	{
 	    e -= 4;
-	    for (s = e; s > match; mb_ptr_back(match, s))
+	    for (s = e; s > match; MB_PTR_BACK(match, s))
 		if (s < match || vim_ispathsep(*s))
 		    break;
 	    ++s;