Fixes#8.
`nebula-cert ca` now supports encrypting the CA's private key with a
passphrase. Pass `-encrypt` in order to be prompted for a passphrase.
Encryption is performed using AES-256-GCM and Argon2id for KDF. KDF
parameters default to RFC recommendations, but can be overridden via CLI
flags `-argon-memory`, `-argon-parallelism`, and `-argon-iterations`.
This fixes a couple issues:
- NoSuchFileError not defined for darwin.
- ca_test and sign_test do a bunch of filesystem specific tests that
error differently on Windows. Just disable these tests on Windows for
now.
- Make the signcert test more deterministic by only testing one existing
file at a time.
