fix decompression with -o writing into a block device

fix decompression with -o writing into a block device

decompression features automatic support of sparse files,
aka a form of "compression" where entire blocks consists only of zeroes.
This only works for some compatible file systems (like ext4),
others simply ignore it (like afs).

Triggering this feature relies of `fseek()`.
But `fseek()` is not compatible with non-seekable devices, such as pipes.
Therefore it's disabled for pipes.

However, there are other objects which are not compatible with `fseek()`, such as block devices.

Changed the logic, so that `fseek()` (and therefore sparse write) is only automatically enabled on regular files.

Note that this automatic behavior can always be overridden by explicit commands `--sparse` and `--no-sparse`.

fix #3583