Make initramfs-tools script encryption aware

Make initramfs-tools script encryption aware

Changed decrypt_fs zfs command to "load-key"
Plymouth case code based on "contrib/dracut/90zfs/zfs-lib.sh.in"
Systemd case based on "contrib/dracut/90zfs/zfs-load-key.sh.in"
Cleaned up misspelling of "available" throughout

Code style fixes
Single quote for ${ENCRYPTIONROOT}
Changed "${DECRYPT_CMD}"  to "eval ${DECRYPT_CMD}"

Reviewed-by: Kash Pande <kash@tripleback.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Garrett Fields <ghfields@gmail.com>
Closes #8093