kernel32: WideCharToMultiByte: return error on negative dest len (take 2)

[Dan Kegel]

While investigating a crash in gs-auftrag in bug 9039, noticed
WideCharToMultiByte wasn't quite conformant for negative destlen.
Fixed, with tests.  Passes for me in Wine and Win XP.

This time I test more than just one negative length,
I also test very long strings, and I removed one stray
#include that snuck in last time.
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