New keyword argument for SB-C:DEFKNOWN: :overwrite-fndb-silently

New keyword argument for SB-C:DEFKNOWN: :overwrite-fndb-silently

 While not useful in the compiler itself, some libraries (not only
 contribs) define their own VOPs, transforms, etc., and the error
 when clobbering pre-existing defknowns made these libraries hard
 to reload.

 Use with :allow-other-keys for backward compatibility.