extension checking is too strict

I was surprised dropping snzip in with other compressors that it *enforces* filename extensions.

Thus:

echo foo | snzip -c >foo.sz; snzcat foo.sz

works. But

echo foo | snzip -c >foo.sz; mv foo.sz foo.sz~; snzcat foo.sz~

fails with error "foo.sz~ has unknown suffix".
However

snzcat