Make "xz --decompress --stdout --force" copy unrecognized

Make "xz --decompress --stdout --force" copy unrecognized
files as is to standard output.

This feature is needed to be more compatible with gzip's
behavior. This was more complicated to implement than it
sounds, because the way liblzma is able to return errors with
files of only a few bytes in size. xz now has its own file
type detection code and no longer uses lzma_auto_decoder().