[krbdev.mit.edu #6973] error reporting made worse in gss_acquire_creds

[Greg Hudson via RT]

Looking into this:

1. gss_acquire_cred tries all of the mechs in sequence and, if none of 
them worked, returns the last major/minor status it saw.  The last mech 
is SPNEGO, which tries all of the other mechs and comes up with a minor 
status of ERR_SPNEGO_NO_MECHS_AVAILABLE, which conveys no useful 
information.

2. spnego_gss_acquire_cred has a bug where it stomps its own minor 
status with a call to gss_release_oid_set(), so we don't even see the 
useless error message; instead we get a minor code which maps to 0 and 
gss_display_status returns the empty string.

I will fix #2 but not link it to this report, since it solves the 
uninteresting part of the problem.  #1 is harder; it's fundamentally 
difficult to return a useful error message for "I tried a bunch of 
things which I don't have a lot of specific knowledge about, and they 
all failed."  Returning the first major/minor code we see would probably 
be more helpful for krb5 use cases, but not for Moonshot or similar use 
cases.