add sigio support

extend code to cope with edge triggered event notification
add io_eagain() so that you can notify io_wait if accept() returned EAGAIN

io/io_trywrite.c

@@ -42,6 +42,12 @@ int64 io_trywrite(int64 d,const char* buf,int64 len) {
   }
   if (r==-1 || r==0) {
     e->canwrite=0;
+#ifdef HAVE_SIGIO
+    if (d==alt_firstwrite) {
+      debug_printf(("io_trywrite: dequeueing %ld from alt write queue (next is %ld)\n",d,e->next_write));
+      alt_firstwrite=e->next_write;
+    }
+#endif
     e->next_write=-1;
   }
   return r;