Disallow leading underscore in option names

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@33579 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/gis/parser.c

@@ -2128,7 +2128,7 @@ static int is_option(const char *string)
 {
     int n = strspn(string, "abcdefghijklmnopqrstuvwxyz0123456789_");
 
-    return n > 0 && string[n] == '=' && string[n-1] != '_';
+    return n > 0 && string[n] == '=' && string[0] != '_' && string[n-1] != '_';
 }
 
 static int set_option(char *string)