*: replace select-for-one descriptor with poll, it's smaller.

$ ./.cmk bloatcheck
function                                             old     new   delta
readit                                               406     364     -42
syslogd_main                                        1249    1206     -43
traceroute_main                                     4115    4060     -55
mysleep                                              112      45     -67
arpping                                              579     441    -138
tftp                                                1575    1182    -393
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/6 up/down: 0/-738)           Total: -738 bytes
   text    data     bss     dec     hex filename
 770580    1051   10764  782395   bf03b busybox_old
 769820    1051   10764  781635   bed43 busybox_unstripped

shell/ash.c

@@ -11589,6 +11589,7 @@ readcmd(int argc, char **argv)
 #endif
 #if ENABLE_ASH_READ_TIMEOUT
 	if (ts.tv_sec || ts.tv_usec) {
+// TODO: replace with poll, it is smaller
 		FD_ZERO(&set);
 		FD_SET(0, &set);