Maps ZPOOL_PROP_AUTOTRIM to PoolPropAutotrim (#20)
Maps ZPOOL_PROP_AUTOTRIM to PoolPropAutotrim (#20)

ZPOOL_PROP_AUTOTRIM is part of Trim support added in zfs 0.8
(https://github.com/zfsonlinux/zfs/commit/1b939560be5c51deecf875af9dada9d094633bf7).
Add it and prevents panic when pool is reloading its properties.
Fixes #19
bicomsystems/go-libzfs,bicomsystems/go-libzfs