list -o props should be alloc,free not used,avail

Manpage suggests the zpool list properties include 'used'
and 'available', when these are invalid property names.
Use alloc and free in their place.

```
$ zpool list -o name,size,used   2>&1 |head -1
bad property list: invalid property 'used'
$ zpool list -o name,size,avail   2>&1 |head -1
bad property list: invalid property 'avail'
$ zpool list -o name,size,available   2>&1 |head -1
bad property list: invalid property 'available'
$ zpool list -o name,size,alloc,free
NAME    SIZE  ALLOC   FREE
apool   464M   203M   261M
bpool  3.62T  1.97T  1.65T
```

Reviewed-by: Giuseppe Di Natale <[email protected]>
Reviewed-by: loli10K <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tom Matthews <[email protected]>
Closes #5959 

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