Add -l option to gssd to force legacy behaviour

Implement a new option -l to force gssd to ignore its kernel's crypto
capabilities and use just the Single DES legacy encryption types to be
compatible with old servers. This is only relevant if those servers have
strong keys in their keytab.

Signed-off-by: Steve Dickson <steved@redhat.com>
Tested-by: Michael Weiser <weiser@science-computing.de>