Add route attribute gw_mpls_stack to make MPLS stack of route nexthop
accessible from filters. Its type is T_CLIST, which is really not correct
(as it is a list, while T_CLIST is a set). Therefore, we keep this
attribute *undocumented* and it will be *changed* without further notice.

Based on a patch from Trisha Biswas <tbiswas@fastly.com>, thanks!