MEDIUM: threads/fd: Initialize the process mask during the call to fd_insert

Listeners will allow any threads to process the corresponding fd. But for other
FDs, we limit the processing to the current thread.
diff --git a/src/dns.c b/src/dns.c
index 2d684b0..c2b87c0 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -193,7 +193,7 @@
 	dgram->t.sock.fd = fd;
 	fdtab[fd].owner  = dgram;
 	fdtab[fd].iocb   = dgram_fd_handler;
-	fd_insert(fd);
+	fd_insert(fd, (unsigned long)-1);
 	fd_want_recv(fd);
 	return 0;
 }