GSS: make Curl_gss_log_error more verbose

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