patch 8.0.0519: character classes are not well tested

Problem:    Character classes are not well tested. They can differ between
            platforms.
Solution:   Add tests.  In the documentation make clear which classes depend
            on what library function.  Only use :cntrl: and :graph: for ASCII.
            (Kazunobu Kuriyama, Dominique Pelle, closes #1560)
            Update the documentation.
diff --git a/src/regexp.c b/src/regexp.c
index 91b8015..b4fe7d7 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -2555,17 +2555,17 @@
 				regc('\t');
 				break;
 			    case CLASS_CNTRL:
-				for (cu = 1; cu <= 255; cu++)
+				for (cu = 1; cu <= 127; cu++)
 				    if (iscntrl(cu))
 					regmbc(cu);
 				break;
 			    case CLASS_DIGIT:
-				for (cu = 1; cu <= 255; cu++)
+				for (cu = 1; cu <= 127; cu++)
 				    if (VIM_ISDIGIT(cu))
 					regmbc(cu);
 				break;
 			    case CLASS_GRAPH:
-				for (cu = 1; cu <= 255; cu++)
+				for (cu = 1; cu <= 127; cu++)
 				    if (isgraph(cu))
 					regmbc(cu);
 				break;