sync. experimental with  trunk

git-svn-id: https://foundry.supelec.fr/svn/luatex/branches/experimental@4737 0b2b3880-5936-4365-a048-eb17d2e5a6bf
Fixed a bug in lua_nodelib_direct_traverse (Hans)

git-svn-id: https://foundry.supelec.fr/svn/luatex/branches/experimental@4739 0b2b3880-5936-4365-a048-eb17d2e5a6bf
Fixed lua_nodelib_direct_copy for a glue of 0pt

git-svn-id: https://foundry.supelec.fr/svn/luatex/branches/experimental@4741 0b2b3880-5936-4365-a048-eb17d2e5a6bf