CLI/show: warn if crypto options are used with mbox format

CLI/show: warn if crypto options are used with mbox format

This limitation seems somewhat hard to fix, but at least try to warn
users when combining crypto operations with mbox output format.

Because the default is --decrypt=auto, the warning is omitted if
--decrypt=auto is specified. While this is not great, it seems more
wrong to always warn, or to change the default because of this.