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 <dinatale2@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Matthews <tom@axiom-partners.com>
Closes#5959
d456708525