Left-align index props

Index type props display as strings, which should be aligned to the
left not to the right.

Before:

FreeBSD-13_0-CURRENT-r356528 ➜  ~ zfs list -ro name,aclmode,mountpoint
NAME        ACLMODE  MOUNTPOINT
p0      passthrough  /p0
p0/foo      discard  /p0/foo

After:

FreeBSD-13_0-CURRENT-r356528 ➜  ~ zfs list -ro name,aclmode,mountpoint
NAME    ACLMODE      MOUNTPOINT
p0      passthrough  /p0
p0/foo  discard      /p0/foo

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #9912