Ported to Linux

- Removed FreeBSD version check
- Manipulations of zpool.cache were changed from /boot/zfs/zpool.cache
  to /etc/zfs/zpool.cache
- Removed adjustments to loader.conf and loader.conf.local
- Changed path of config file from /usr/local/etc/beadm.conf to /etc/beadm.conf
- Changed to GNU date command
- When activating a dataset, set mountpoint to legacy rather than "/"
- Adjusted beadm mount command to return temporary mount path for use
with scripting
- Added some testing and porting details to README.md