Add a consideration of the number of key slot

[suikan4github]

The number of the key slot in LUKS volume is monotonically increasing. Even overwrite installation increase the number. Thus, some day, user cannot install new distribution, even he has less than 7 distributions in a LUKS volume.

[suikan4github]

Followings is the command to check which slot is assigned for given key file.

sudo cryptsetup open -v --test-passphrase --key-file boot_os.keyfile /dev/sda2

[suikan4github]

Merged to develop. Ready to reelase.