Introduce the 'n' flag for the geli attach command.

If the 'n' flag is provided the provided key number will be used to
decrypt device. This can be used combined with dryrun to verify if the key
is set correctly. This can be also used to determine which key slot we want to
change on already attached device.

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D15309