Support USE_DCE_STYLE

[jborean93]

In order to use this library for RPC authentication it needs to support USE_DCE_STYLE which does a three-leg authentication process as needed by RPC.

[awakecoding]

The ISC_REQ_USE_DCE_STYLE flag is set for the RPC bind, but this flag is not implemented, which may explain why FreeRDP + RDG (RPC-based) fails right now in EncryptMessage:

[jborean93]

Just an FYI #120 is also related to RPC/DCE support. It is needed for using rpc_protect_level_pkt_privacy (encrypted payloads) which some RPC endpoints mandate.

[TheBestTvarynka]

implemented in #381