make io_waituntil actually take an "until", not a "how long"

fix io_timeouted to ignore fds where no timeout has been set

CHANGES

@@ -5,6 +5,7 @@
   the integer scan routines should only write *dest if they actually
     scanned something
   io_sendfile and iob_send should return -1 to -3 just like io_trywrite
+  make io_waituntil actually take an "until", not a "how long"
 
 0.16:
   add buffer_fromsa (make buffer from stralloc)

io/io_timeouted.c

@@ -10,8 +10,9 @@ int64 io_timeouted() {
   if (ptr>=alen) ptr=0;
   e=array_get(&io_fds,sizeof(io_entry),ptr);
   if (!e) return -1;
-  for (;ptr<alen; ++ptr,++e)
+  for (;ptr<alen; ++ptr,++e) {
-    if (e->inuse && taia_less(&e->timeout,&now))
+    if (e->inuse && e->timeout.sec.x && taia_less(&e->timeout,&now))
       return ptr;
+  }
   return -1;
 }

io/io_waituntil.c

@@ -7,9 +7,10 @@
 
 void io_waituntil(tai6464 t) {
   uint64 x,y;
-  tai6464 now;
+  tai6464 now,diff;
   taia_now(&now);
-  if (!umult64(now.sec.x,1000,&x) || (y=x+now.nano/10000000)<x)
+  taia_sub(&diff,&t,&now);
+  if (!umult64(diff.sec.x,1000,&x) || (y=x+diff.nano/10000000)<x)
     y=-1;	/* overflow; wait indefinitely */
   io_waituntil2(y);
 }