[krbdev.mit.edu #5896] improve thread safety of gss wrap/unwrap functions

[Ken Raeburn via RT]

As discussed recently on both kerberos at mit and krbdev at mit, it'd be nice if the wrap/unwrap 
type functions could operate in multiple threads without application-level locking.

Specific use cases:

* multiple threads operating on messages on an unsequenced message stream

* a sequenced message stream treated as full-duplex with two threads (e.g., http://www.openldap.org/lists/openldap-technical/200802/msg00121.html)

Oh, and document it.

The krb5 mechanism data includes two sequence numbers, a krb5_context, and a 
krb5_auth_context.