Prior to this change, we were generating the SPN in the SSPI code when
the credentials were NULL and in the GSS-API code when the context was
empty. It is better to decouple the SPN generation from these checks
and only generate it when the SPN itself is NULL.
This also brings this part of the Kerberos 5 code in line with the
Negotiate code.
For consistency with the spnego and oauth2 code moved the setting of
the host name outside of the Curl_auth_create_gssapi_user_messag()
function.
This will allow us to more easily override it in the future.
Also display the GSS_C_GSS_CODE (major code) when specified instead of
only GSS_C_MECH_CODE (minor code).
In addition, the old code was printing a colon twice after the prefix
and also miscalculated the length of the buffer in between calls to
gss_display_status (the length of ": " was missing).
Also, gss_buffer is not guaranteed to be NULL terminated and thus need
to restrict reading by its length.
Closes#738
