[tlbuild] adding scripts

[Karl Berry]

    But that is only for symlinked scripts, isn't it?  Shell stub-like
    scripts only need to go under Master/bin/*/ and that's it I think (but
    correct me if I'm wrong).

That is wrong.  The guiding principle is that the *entire* bin dir is
created by running a Build.

(The one exception is asy.  And it's a big pain to have any exceptions,
but there's no alternative.)

We can install links (using linked_scripts), or actual scripts (by
putting them into some other directory and "make install"ing them), but
they have to be under Build somewhere.

What I did with them previously (putting them under linked_scripts) was
wrong, hence the broken links that Angelo noted.  That'll be fixed in
the next round of builds, but it's not quite time for that yet.

I'll discuss the context scripts specifically further with Taco and Mojca.