fscrypt-crypt-util: use an explicit --direct-key option

fscrypt-crypt-util: use an explicit --direct-key option

Make fscrypt-crypt-util use an option --direct-key to specify the use of
the DIRECT_KEY method for key derivation and IV generation.  Previously,
this method was implicitly detected via --mode-num being given without
either --iv-ino-lblk-64 or --iv-ino-lblk-32, or --kdf=none being given
in combination with --file-nonce.

The benefit of this change is that it makes the various options to
fscrypt-crypt-util behave more consistently.  --direct-key,
--iv-ino-lblk-64, and --iv-ino-lblk-32 now all work similarly (they
select a key derivation and IV generation method); likewise, --mode-num,
--file-nonce, --inode-number, and --fs-uuid now all work similarly (they
provide information that key derivation and IV generation may need).

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>