Change the type declaration of the return value for %iconv
It used to be :INT, but the function actually returns a size_t, and this
can cause problems on some 64-bit platforms. A much better type to use
is :UNSIGNED-LONG (as mentioned here, for example:
http://common-lisp.net/project/cffi/manual/html_node/Tutorial_002dCallbacks.html).

This change also allows us to simplify the computation of +ERROR-RETURN+, which
in turns allows us to remove the EVAL-WHEN construct surrounding the declarations
of the CFFI functions.