crypto: change network packets and add dynamic crypto header/data

The new network packet will look:

struct crypto_config_header * that provides info on crypto/hashing
hash_block[size based on hashing function] (if hash is selected)
salt[SALT_SIZE] (if crypto is selected)
...data...

and we kill the concept of crypto_security_header completely since
values are now dynamic for hash_block_size.

the reason why hash_block needs to be there, is because we do
hash salt in case both hashing and crypto are selected.

the crypto_config_header is totally transparent to totem
and to any underlaying crypto functions.

as we go cleaning, also use HASH_BLOCK_SIZE to generate hash_block.
the input buffer and output buffer size are dependent on the algo
used to hash.

we can now determine the real header size and adjust net_mtu properly
at startup. This will allow in future to use any algorithm since
size is dynamic.

some part of the code still needs some polishing to make it more
readable (specially the mapping of pointers into the packet
is still a bit obscure).

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>