Currently, zfs(8) and zpool(8) print (null) if you pass in invalid
types, sources or column names for zfs get, zfs list and `zpool
get`. This is because the commands use getsubopt(3), and in case of
failure, they print value, which is NULL when sub options don't match.

They should print suboptarg instead, which is the documented way to
get at the non-matching sub option value.

I'm unsure whether this should be sent upstream, as I don't know whether
OpenSolaris or IllumOS have the suboptarg variable.