(a) auth_refresh recursively calls clnt_call() which will call
clnt_vc_call() and will try to acquire a lock on the socket which we have
already acquires. thus a change to see if the thread trying to acquire the
lock is the same holding the lock.
(b) authsspi_fresh() needed to check if we were called to refresh the
context due to the error (ie 2nd argument non-null) and if so, destroy
the old context and then reacquire a new sspi context.
it seems that InitializeSecurityContext() also requires new creds as well
so after initially calling AcquireCreds() we don't need to worry about
refreshing credentials.
