Use newly added keytab functions

Use the new functions added in the previous patch.

Obtain machine credentials in a pre-determined order

Look for appropriate machine credentials in the following order:
     root/<fqdn>@REALM
     nfs/<fqdn>@REALM
     host/<fqdn>@REALM
     root/<any-name>@REALM
     nfs/<any-name>@REALM
     host/<any-name>@REALM

The first matching credential will be used.

Also, the machine credentials to be used are now determined
"on-demand" rather than at gssd startup.  This allows keytab
additions to be noticed and used without requiring a restart of gssd.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>