patch 8.1.0512: 'helplang' default is inconsistent for C and C.UTF-8

Problem:    'helplang' default is inconsistent for C and C.UTF-8.
Solution:   Don't accept a value unless it starts with two letters.
diff --git a/src/version.c b/src/version.c
index 8440ce9..3639702 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    512,
+/**/
     511,
 /**/
     510,