support accents in mountpoint `add-protector-to-policy` subcommand

[an-ca]

Hello,

The add-protector-to-policy fails with the é character for instance:

[ERROR] fscrypt metadata add-protector-to-policy: flag value
                                                  "/mnt/miroir-données:d77aa788ff9d1931"
                                                  does not have format MOUNTPOINT:ID

whereas the setup command allows it. Works if é is replace by e.

I haven't checked on all other commands and subcommands.

Cheers,
Anthony