Add ShellCheck's --enable=all inside cmd/

The only exception is cmd/vdev_id/vdev_id which might be a subject of
refactoring (see #12084)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12912