gssd: validate cred in gssd_acquire_user_cred

Call gss_inquire_cred after gssd_acquire_krb5_cred check for expired
credentials.

This fixes a recent regression (since 302de786930a2c533068f9d8909a)
that causes the user's ticket cache to grow unbounded with expired
service tickets when the user's credentials expire.

To reproduce this issue:

 - mount kerberos nfs export
 - kinit for a short lifetime (ie "kinit -l 1m")
 - run a job that opens a file and writes for more than the lifetime
 - run klist a few times after expiry and see the list grow, ie:

Ticket cache: DIR::/run/user/1749600001/krb5cc/tktYmpGlX
Default principal: dros@APIKIA.FAKE

Valid starting       Expires              Service principal
10/21/2013 15:39:38  10/21/2013 15:40:35  krbtgt/APIKIA.FAKE@APIKIA.FAKE
10/21/2013 15:39:40  10/21/2013 15:40:35 nfs/zero.apikia.fake@APIKIA.FAKE

Signed-off-by: Weston Andros Adamson <dros@netapp.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>
exportfs: report failure if asked to unexport something not exported.

Currently if exportfs is asked to unexport something that is not
exported it silently succeeds.  This is not ideal, particularly for
scripting situations.

So report an error when the unexport was successful and the -v flag used.

Signed-off-by: Steve Dickson <steved@redhat.com>