Authenticates the request message.
    Format
      #include  <rpc/rpc.h>
      enum auth_stat  _authenticate(struct svc_req *rqst, struct
                      rpc_msg *msg);

1  –  Arguments

 rqst
    A pointer to an svc_req structure with the requested program
    number, procedure number, version number, and credentials passed
    by the client.
 msg
    A pointer to an rpc_msg structure with members that make up the
    RPC message.

2  –  Description

    Returns AUTH_OK if the message is authenticated successfully. If
    it returns AUTH_OK, the routine also does the following:
    o  Sets rqst->rq_xprt->verf to the appropriate response verifier.
    o  Sets rqst->rq_client_cred to the "cooked" form of the
       credentials.

    The expression rqst->rq_xprt->verf must be preallocated and its
    length must be set appropriately.

    The program still owns and is responsible for msg->u.cmb.cred and
    msg->u.cmb.verf. The authentication system retains ownership of
    rqst->rq_client_cred, the "cooked" credentials.

3  –  Return Values

    enum auth_stat     The return status code for the authentication
                       checks:
                          AUTH_OK=0-Authentication checks successful.
                          AUTH_BADCRED=1-Invalid credentials (seal
                          broken)
                          AUTH_REJECTEDCRED=2-Client should begin new
                          session
                          AUTH_BADVERF=3-Invalid verifier (seal
                          broken)
                          AUTH_REJECTEDVERF=4-Verifier expired or was
                          replayed
                          AUTH_TOOWEAK=5-Rejected for security
                          reasons
                          AUTH_INVALIDRESP=6-Invalid response
                          verifier
                          AUTH_FAILED=7-Some unknown reason