nfs4.1: Minimal SP4_MACH_CRED implementation

This is a minimal client side implementation of SP4_MACH_CRED.  It will
attempt to negotiate SP4_MACH_CRED iff the EXCHANGE_ID is using
krb5i or krb5p auth.  SP4_MACH_CRED will be used if the server supports the
minimal operations:

 BIND_CONN_TO_SESSION
 EXCHANGE_ID
 CREATE_SESSION
 DESTROY_SESSION
 DESTROY_CLIENTID

This patch only includes the EXCHANGE_ID negotiation code because
the client will already use the machine cred for these operations.

If the server doesn't support SP4_MACH_CRED or doesn't support the minimal
operations, the exchange id will be resent with SP4_NONE.

Signed-off-by: Weston Andros Adamson <dros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>