Support encrypted offline master keys with a new --keygen flag

When --keygen is provided, we prompt for a passphrase when we make a
new master key; if it is nonempty, we store the secret key in a new
crypto_pwbox.

Also, if --keygen is provided and there *is* an encrypted master key,
we load it and prompt for a passphrase unconditionally.

We make a new signing key unconditionally when --keygen is provided.
We never overwrite a master key.