Add new retry filter.

Add new retry filter.

This filter can be used to transparently reopen/retry when a plugin
fails.  The connection is closed and reopened which for most plugins
causes them to attempt to reconnect to their source.

For example if doing a long or slow SSH copy:

  nbdkit -U - ssh host=remote /var/tmp/test.iso \
    --run 'qemu-img convert -p -f raw $nbd -O qcow2 test.qcow2'

if the SSH connection or network goes down in the middle then the
whole operation will fail.

By adding the retry filter:

  nbdkit -U - ssh --filter=retry host=remote /var/tmp/test.iso \
    --run 'qemu-img convert -p -f raw $nbd -O qcow2 test.qcow2'

this operation can recover from temporary failures in at least some
circumstances.  The NBD connection (a local Unix domain socket in the
example above) is not interrupted during retries, so NBD clients don't
need to be taught how to retry - everything is handled internally by
nbdkit.