Revision 333438

Change option dry-run from 'n' to 'C' in geli attach command.

'n' is used in other commands to define the key index.
We should be consistent with that.
'C' option is used by patch(1) to perform dryrun so lets use that.

Reviewed by:	allanjude
Differential Revision:	https://reviews.freebsd.org/D15308