Restricting the `zfs list` command to depth 1 saves a lot of time for
pools with many datasets/zvols.
In my case, before:
```
$ time zfs list -s name -o name,guid,available -H -p >/dev/null
real 0m3.853s
user 0m0.171s
sys 0m3.539s
```
After:
```
$ time zfs list -d 1 -s name -o name,guid,available -H -p >/dev/null
real 0m0.027s
user 0m0.002s
sys 0m0.026s
```
Signed-off-by: Joel Low <joel@joelsplace.sg>
