note to self on ZOL mountall

To make this work on Ubuntu, manually call `zpool import` before
upstart gets any of the "*-filesystems" events.  Adding an exec line
for each pool to the `/etc/init/mountall.conf` file should get the
desired result:

script
  # ...
  exec zpool import -N alfa
  exec zpool import -N bravo
  exec zpool import -N charlie
  exec mountall --daemon $force_fsck $fsck_fix
end script

The -N switch matters here, and the patched mountall from the PPA is
still required.

-Darik