Silence invalid parameter messages from win32_signal

Silence invalid parameter messages from win32_signal

This is the first step towards enabling the invalid parameter handler
without it causing undue noise. In this case the invalid parameters are
intentional, so provide a means to silence messages about them.

There is still noise from win32_close() and win32_select() which needs
resolving by some means too before the handler can be switched on without
its output causing test failures.