If a caller passes an empty mech set to gss_acquire_cred, get a cred
for all mechs instead of just the krb5 mech, as we don't know what
mechanism the cred is going to be used with (particularly in the
acceptor case). As a related fix, if a caller passes a credential to
gss_accept_sec_context and it does not contain a mech-specific cred
for the token's mech, error out instead of using the default cred with
the token's mechanism.
Commit By: ghudson