/rpool--var--log -> /rpool-var-log using systemd-style escaping

[wdoekes]

# systemd-escape rpool/var/log/this-is-a-test
rpool-var-log-this\x2dis\x2da\x2dtest

That should be more readable. And it's nice to use a consistent, reversible and documented format.

[wdoekes]

7a20e8b