Multiple versions of Lua with alternatives

[Lemures Lemniscati]

Hi!

We have lua5.1 packages [1] besides lua packages [2] in the current
Cygwin distribution, although we have no lua 5.1 module packages.

So I've added simply lua51 (obsoleting lua5.1) and no lua51-* module
to the tree [3], which I'm proposing.

[1]: https://www.cygwin.com/packages/summary/lua5.1-src.html
[2]: https://www.cygwin.com/packages/summary/lua-src.html
[3]: https://cygwin-lem.github.io/lua-cygwin-packages/

Regards,

Lem