s3-rpc: Decrypt with the proper session key in CreateTrustedDomainEx2.

s3-rpc: Decrypt with the proper session key in CreateTrustedDomainEx2.

On LSA and SAMR pipes session_key is truncated to 16 byte when doing encryption/decryption.
However, this was not done for trusted domain-related modifying operations.

As result, Samba 4 client libraries do not work against Samba 3 while working
against Windows 2008 r2.

Solved this by introducing "session_extract_session_key()" function that allows to specify
intent of use of the key.

Signed-off-by: Andreas Schneider <asn@samba.org>
Autobuild-User: Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date: Tue Mar 13 12:23:44 CET 2012 on sn-devel-104