We used to do validation using lists of mandatory feature flags.
With the introduction of XChaCha20Poly1305, this became too
simplistic, as it uses a different IV length, hence disabling
GCMIV128.
Add a dedicated function, Validate(), with open-coded validation
logic.
The validation and creation logic also gets XChaCha20Poly1305
support, and gocryptfs -init -xchacha now writes the flag into
gocryptfs.conf.
