Local /iocage/jails/<uuid>/(prestart|poststart|prestop|poststop) overrides zfs properties.

If an executable file "prestart|poststart|prestop|poststop" is present
in the jail mountdir, it will be used instead of the command configured
in the corresponding exec_* jail property.
Merge pull request #8 from abochem/master

Default for prestart|poststart|prestop|poststop commands