udisks2: Change display name for crypto_unknown devices

The udisks id_type crypto_unknown is used for devices which are possibly encrypted (which was introduced to allow unlocking TCRYPT volumes, which are indistinguishable from random data).

In udisks, this uncertainty is conveyed to the user by using the long name "Possibly encrypted", and the short name "Encrypted?" (see https://github.com/storaged-project/udisks/blob/ab74f67917ea2a8c2b18dec0d5dcb7b33b80a967/udisks/udisksclient.c#L1930).

In gvfs, all devices with id_usage == "crypto" get displayed as "%s Encrypted" (where %s is the formatted size). This instead uses the string "Possibly Encrypted" if id_type == "crypto_unknown".

Closes #412 (closed)