Introduce lukslabel option + fix for is_active_luks_mapping()

When "lukslabel" is set for persistence-encryption then live-boot only tries to open containers that have the label "persistence" set. It takes precedence over the "luks" option.

The behavior of the "luks" option is to prompt the user to open any LUKS container. This was required for LUKS1 because it did not support labels, but LUKS2 does. On systems were there are multiple encrypted containers this is not ideal, because for users it is not clear why they should decrypt their internal hard drive to boot a live system.