Fixes regression in dfd8a5f that made autoload not add more files
at the end of the current playlist if playlist was made by the
script.
This still prevents loading the script if more than one (media or
playlist) file was manually added.
Commit e2e450f9 started making use of luaL_register(), but OF COURSE
this function disappeared in Lua 5.2, and was replaced with a 5.2-only
alternative, slightly different mechanism.
So just NIH our own function. This is actually slightly more correct,
since it forces the user to call "require" to actually make the module
visible for builtin C-only modules other than "mp". Fix autoload.lua
accordingly.
This will load other files in the same directory when a single file is
played. It's an often requested feature, but we definitely don't want it
in the core.
