Subject: SPNEGO can't display mechanism errors

If SPNEGO makes a call to an underlying mechanism and it fails, SPNEGO
passes the minor status it received back down into the mechglue.  The
mechglue maps the mech OID and status to a new value (like 100005) and
returns it to the app.

When the app calls gss_display_status on this value, the mechglue
faithfully unmaps it to SPNEGO and the underlying minor code, and calls
spnego_gss_display_status.  But that function only knows how to display
SPNEGO-specific error codes, and returns an empty string for anything
else.

The simple solution would be for spnego_gss_display_status to
recursively call gss_display_status on unrecognized codes.  That
solution runs the risk of blowing the stack if, for some reason,
gss_display_status winds up calling back into spnego_gss_display_status
with the same code.  There are ways to eliminate this risk (a thread-
specific variable remembering if we've already called into
spnego_gss_display_status, or a SPNEGO-spceific minor status map), but
they're complicated.