mime-node: rename decrypted_child to unwrapped_child
When walking the MIME tree, we might need to extract a new MIME
object. Thus far, we've only done it when decrypting
multipart/encrypted messages, but PKCS#7 (RFC 8551, S/MIME) has
several other transformations that warrant a comparable form of
unwrapping.
Make this member re-usable for PKCS#7 unwrappings as well as
multipart/encrypted decryptions.
This change is just a naming change, it has no effect on function.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>