Check that the gssapi library is usable early on.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Do a call to determine mechanisms supported by the gssapi library early.
This allows us to discover early in case the gssapi library is somehow
misconfigured.  We can bail out early and give a meaningful message
rather than getting errors on each attempt at a context negotiation.