Don't use salt for encryption

The openssl-enc documentation says "This option SHOULD NOT be used ...".

I claim that -nosalt is safe to use in this application because:

1. The secrets being used here are full key-size + IV-size random values
   straight out of the random number generator.
2. Keys are never re-used.

-nosalt is desirable for this application because it avoids the "Salted__..."
header.  Having a "Salted__" header on every single layer of encryption
allows an attacker to get a known-plaintext-attack fingernail under each
layer, giving an opportunity to attack each layer individually.