patch 8.1.0539: cannot build without the sandbox

Problem:    Cannot build without the sandbox.
Solution:   Set the secure option instead of using the sandbox.  Also restrict
            the characters from 'spelllang' that are used for LANG.vim.
            (suggested by Yasuhiro Matsumoto)

runtime/doc/options.txt

@@ -7339,7 +7339,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 
 	After this option has been set successfully, Vim will source the files
 	"spell/LANG.vim" in 'runtimepath'.  "LANG" is the value of 'spelllang'
-	up to the first comma, dot or underscore.
+	up to the first character that is not an ASCII letter and not a dash.
 	Also see |set-spc-auto|.
 
 

src/buffer.c

@@ -5523,11 +5523,11 @@ chk_modeline(
 		current_sctx.sc_lnum = 0;
 #endif
 		// Make sure no risky things are executed as a side effect.
-		++sandbox;
+		++secure;
 
 		retval = do_set(s, OPT_MODELINE | OPT_LOCAL | flags);
 
-		--sandbox;
+		--secure;
 #ifdef FEAT_EVAL
 		current_sctx = save_current_sctx;
 #endif

src/option.c

@@ -7758,10 +7758,13 @@ did_set_string_option(
 	     * '.encoding'.
 	     */
 	    for (p = q; *p != NUL; ++p)
-		if (vim_strchr((char_u *)"_.,", *p) != NULL)
+		if (!ASCII_ISALPHA(*p) && *p != '-')
 		    break;
-	    vim_snprintf((char *)fname, 200, "spell/%.*s.vim", (int)(p - q), q);
+	    if (p > q)
-	    source_runtime(fname, DIP_ALL);
+	    {
+		vim_snprintf((char *)fname, 200, "spell/%.*s.vim", (int)(p - q), q);
+		source_runtime(fname, DIP_ALL);
+	    }
 	}
 #endif
     }

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    539,
 /**/
     538,
 /**/