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

include/asterisk/linkedlists.h

@@ -429,6 +429,23 @@ struct {								\
       }									\
 } while (0)
 
+/*!
+  \brief Appends a whole list to the tail of a list.
+  \param head This is a pointer to the list head structure
+  \param list This is a pointer to the list to be appended.
+  \param field This is the name of the field (declared using AST_LIST_ENTRY())
+  used to link entries of this list together.
+ */
+#define AST_LIST_APPEND_LIST(head, list, field) do {			\
+      if (!(head)->first) {						\
+		(head)->first = (list)->first;				\
+		(head)->last = (list)->last;				\
+      } else {								\
+		(head)->last->field.next = (list)->first;		\
+		(head)->last = (list)->last;				\
+      }									\
+} while (0)
+
 /*!
   \brief Removes and returns the head entry from a list.
   \param head This is a pointer to the list head structure