Switch from lua_objlen() to Lua 5.2 preferred

lua_rawlen(). Define lua_rawlen() in terms of
lua_objlen() when building against older Lua.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1612552 13f79535-47bb-0310-9956-ffa450edef68