cgdconfig(8): Add an argon2id password-based key generation method

This provides an extra level of side-channel and cracking resistance
compared to the pre-existing pkcs5_pbkdf2/sha1 method used for
password-based disk encryption.

Several new keygen parameters are supported:

memory (integer, in kilobytes)
parallelism (integer, usually the number of CPU cores)
version (integer, usually 19...)

We do our best to calibrate these automatically when the paramsfile
is initially generated.

lgtm riastradh@