Cactus scripting language integration

I wanted to add the capability to call lua from within Cactus, and in particular from schedule.ccl The attached patch does this, but tries to do this as generic as possible, for future implementations of other scripting languages as well.

It should be seen as 'in development'. I am asking for comments. We could develop this separate on a branch, or, of it is agreed that this is "safe enough to not interfere in case a thorn actually using it", we could apply this to trunk after the release.

Use the latest version of ExternalLibraries/lua to test this, but note that there are some hard-coded paths still in there (in the lua thorn), which would have to be generalized first.

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