fscrypt-crypt-util: add support for dumping key identifier

Add an option to fscrypt-crypt-util to make it compute the key
identifier for the given key.  This will allow testing the correctness
of the filesystem's key identifier computation.

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