r4627: - simplified the dcerpc auth code using a common function

r4627: - simplified the dcerpc auth code using a common function

- added support for "spnego" in binding strings. This enables SPNEGO
  auth in the dcerpc client code, using as many allter_context calls as
  are needed

To try SPNEGO do this:

  smbtorture ncacn_ip_tcp:SERVER[spnego,seal] -Uadministrator%password RPC-SAMR