load-library implies that dlopen() is called, which is totally not the
case. add-library calls dlopen() and load-library just looks the opened
library up in the ``libraries`` global. What load-library did is now
library-dll.
If we want to do it right, add-library should be a declarative top-level
form that tries to call a real load-library on demand instead of
memoizing the first try which would fix the case where the library is
not found, is moved into position by the user and Factor declines to
look it up again.
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