use variable for path to zfs binary.

For zfsonlinux on centos, `zfs` is located at `/sbin/zfs` rather than
`/usr/sbin/zfs`. Refer to the binary with a `$ZFS` variable. Since there
is a local and a remote, they may have a different OS so use a `$REMZFS`
for the remote end.
Merge pull request #3 from dnozay/variable

Use variables for local/remote paths to zfs binary.