Skip unnecessary mech calls in gss_inquire_cred()

If the caller does not request a name, lifetime, or cred_usage when
calling gss_inquire_cred(), service the call by copying the mechanism
list (if requested) but do not call into the mech.

This change alleviates an issue (reported by Adam Bernstein) where
SPNEGO can fail in the presence of expired krb5 credentials rather
than proceeding with a different mechanism, or can resolve a krb5
credential without the benefit of the target name.

https://github.com/krb5/krb5/commit/ff5eb892910eeac335d989ae14020da4ffbcc8ec
Author: Greg Hudson <ghudson@mit.edu>
Commit: ff5eb892910eeac335d989ae14020da4ffbcc8ec
Branch: master
 src/lib/gssapi/mechglue/g_inq_cred.c |   41 +++++++++++++++++++---------------
 1 files changed, 23 insertions(+), 18 deletions(-)