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_nfa.c b/src/regexp_nfa.c
index 20ef186..e6d8255 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -4871,7 +4871,7 @@
 		return OK;
 	    break;
 	case NFA_CLASS_CNTRL:
-	    if (c >= 1 && c <= 255 && iscntrl(c))
+	    if (c >= 1 && c <= 127 && iscntrl(c))
 		return OK;
 	    break;
 	case NFA_CLASS_DIGIT:
@@ -4879,7 +4879,7 @@
 		return OK;
 	    break;
 	case NFA_CLASS_GRAPH:
-	    if (c >= 1 && c <= 255 && isgraph(c))
+	    if (c >= 1 && c <= 127 && isgraph(c))
 		return OK;
 	    break;
 	case NFA_CLASS_LOWER: