Fix vndcompress restart failure fallback when input is a pipe.

Defer seeking the *input* image, or winding it forward, until we are
certain we all ready in the cloop2 output, because when the input
image is a pipe, we don't get a chance to seek back to the beginning
and start from the top instead of restarting.

If restart does fail, don't try to seek the input image back to the
beginning unless we had already tried to seek or wind it forward.

Add some automatic tests for this and related cases.

XXX pullup to netbsd-7, netbsd-6
Adopy recent software volume control change.