Point iocroot to /usr/local/iocell

[redrabbit]

Instead of having the root ZFS mountpoint set to /iocell which might not be the most standard way to store files at, this PR sets the default mountpoint to /usr/local/iocell.

See issue #32 for more details.

[redrabbit]

From the source-code, only lib/ioc-globals and lib/ioc-zsh require changes.
Other utility scripts use ${pool}/iocell which is always relative to the ZFS pool initialised with iocell activate $POOL.

[bartekrutkowski]

To make such change accepted, you need to provide not only sufficient testing suite (ideally, for every single subcommand) but also a migration path for users with current version of iocell.

[redrabbit]

This PR should only affect new iocell installations (when running iocell activate $POOL).
It affects the ZFS mounting point when activating the service for the first time. After activation all iocell commands use ${pool}/iocell instead of the absolute path.

I'm not completely sure how lib/ioc-globals is affected when upgrading. Will it change iocroot=/iocell to iocroot=/usr/local/iocell (and break things) or leave it untouched because it detects that both versions diverge?

If I edit the content of /usr/local/lib/iocell/ioc-globals on my local system, when deleting or reinstalling the package, the file is left untouched.