#4400, #5462: Move DH helpers to crypto-utils

On a way to factoring out OpenSSL support to a standalone file to ease
addition of other crypto libraries support in the future, move helpers
providing DH key exchange to crypto-utils.{c,h}. OpenSSL-related
functionality (DH context management) is moved to crypto-utils-openssl.c.
Since we know in advance that DH secret key management code will be the
same for most of backends, implement common functionality in separate
crypto-utils-fallback.c.

Add new tr_dh_ctx_t and tr_dh_secret_t types and functions to be
implemented by crypto backends:
* tr_dh_new - allocate DH context,
* tr_dh_free - free the context,
* tr_dh_make_key - generate private/public keypair,
* tr_dh_agree - perform DH key exchange and generate secret key,
* tr_dh_secret_derive - calculate secret key hash,
* tr_dh_secret_free - free the secret key,
* tr_dh_align_key - align some DH key in the buffer allocated for it.

Make DH secret key not accessible in plain form outside the crypto
backend. This allows for implementations where the key is managed by
the underlying library and is not even exposed to our backend.


git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@14358 f4695dd4-2c0a-0410-b89c-da849a56a58e