Fix a few issues related to the handling of channel variables

- in pbx_builtin_serialize_variables(), the variable list traversal would stop
   on a variables with empty name/values, which is not appropriate
 - When removing the GROUP variables, use AST_LIST_REMOVE_CURRENT instead of
   AST_LIST_REMOVE
 - During masquerading, when copying the variables list from one channel to the
   other, using AST_LIST_INSERT_TAIL is not valid for appending a whole list.
   It leaves the tail pointer of the list invalid.  Introduce a new macro,
   AST_LIST_APPEND_LIST that appends a list properly.
(issue #7802, softins)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.2@40994 65c4cc65-6c06-0410-ace0-fbb531ad65f3

pbx.c

@@ -5906,9 +5906,7 @@ int pbx_builtin_serialize_variables(struct ast_channel *chan, char *buf, size_t
 	memset(buf, 0, size);
 
 	AST_LIST_TRAVERSE(&chan->varshead, variables, entries) {
-		if(variables &&
-		   (var=ast_var_name(variables)) && (val=ast_var_value(variables)) &&
-		   !ast_strlen_zero(var) && !ast_strlen_zero(val)) {
+		if ((var = ast_var_name(variables)) && (val = ast_var_value(variables))) {
 			if (ast_build_string(&buf, &size, "%s=%s\n", var, val)) {
 				ast_log(LOG_ERROR, "Data Buffer Size Exceeded!\n");
 				break;