Readonly mounts - fixes 93

Fixes #93.

Depends on #598; tests should pass once #602 is merged and this branch is merged forward.
Ah, now I have the answer to my earlier question. Fine. Is this to catch cases where the target system doesn't have zfs installed? Wouldn't we be better to have an upfront check for this? And if I try to use this function but mistype the subcommand name,isn't it going to be difficult to debug?