[AC21.4] gnuclient buffer overflow fixes

lib-src/ChangeLog addition: 2006-08-11 Jerry James <james(a)xemacs.org&gt; * gnuslib.c (disconnect_from_server): shutdown() has been fine on Linux for a long time now; use it. Also, don't use length to access the buffer unless it is positive, not just nonzero. * gnuclient.c (filename_expand): Initialize the last array element to get a valid C string in case of overflow. Use strncat to avoid buffer overruns. * gnuclient.c (main): Use strncpy to avoid buffer overruns.