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.
...but keep it disabled by default for new filesystems.
We are still missing an example filesystem and CLI arguments
to explicitely enable and disable it.
This should make things saner and more extensible. It prepares
the infrastructure for "required feature flags" that will be used
to deprecate old gocryptfs version.
