MEDIUM: polling: prepare to call the iocb() function when defined.

We will need this to centralize I/O callbacks. Nobody sets it right
now so the code should have no impact.
diff --git a/src/ev_select.c b/src/ev_select.c
index 64031cc..b1c41bc 100644
--- a/src/ev_select.c
+++ b/src/ev_select.c
@@ -146,19 +146,25 @@
 			/* if we specify read first, the accepts and zero reads will be
 			 * seen first. Moreover, system buffers will be flushed faster.
 			 */
+			fdtab[fd].ev &= FD_POLL_STICKY;
 			if (FD_ISSET(fd, tmp_evts[DIR_RD])) {
 				if (!fdtab[fd].owner)
 					continue;
 				fdtab[fd].ev |= FD_POLL_IN;
-				fdtab[fd].cb[DIR_RD].f(fd);
+				if (fdtab[fd].cb[DIR_RD].f)
+					fdtab[fd].cb[DIR_RD].f(fd);
 			}
 
 			if (FD_ISSET(fd, tmp_evts[DIR_WR])) {
 				if (!fdtab[fd].owner)
 					continue;
 				fdtab[fd].ev |= FD_POLL_OUT;
-				fdtab[fd].cb[DIR_WR].f(fd);
+				if (fdtab[fd].cb[DIR_WR].f)
+					fdtab[fd].cb[DIR_WR].f(fd);
 			}
+
+			if (fdtab[fd].iocb && fdtab[fd].owner && fdtab[fd].ev)
+				fdtab[fd].iocb(fd);
 		}
 	}
 }