patch 7.4.1857

Problem:    When a channel appends to a buffer that is 'nomodifiable' there is
            an error but appending is done anyway.
Solution:   Add the 'modifiable' option.  Refuse to write to a 'nomodifiable'
            when the value is 1.

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1857,
 /**/
     1856,
 /**/