Add release method to service auth ops.

The caller uses SVCAUTH_RELEASE(auth, req) to release any
auth handle, after which it may make no further use of
the handle.

For AUTH_NONE and AUTH_SYS, this operation is a no-op.  For
RPCSEC_GSS, the call is necessary for auth handle reclamation.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>