Require the latest and greatest luaotfload

This simplifies things, and since luaotfload is backward compatible with
old LuaTeX it shouldn’t be an issue for people wanting latest fontspec
to use the latest luaotfload.
Drop code for older luaotfload
Drop LuaTeX exception for multiple alternates

Recent luaotfload now indexes from zero like XeTeX, so we can just use
the same code now.
Always use XeTeX-style name and path lookups

This have been supported by luaotfload for so long.
New release