pkd_daemon.c: force close pkd_state.server_fd upon stop

There's a race window between the accept loop's call to
accept(2) and it checking `ctx.keep_going`.  Forcefully
close the server socket such that any raced `accept` ends
up failing.

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

tests/pkd/pkd_daemon.c

@@ -498,6 +498,7 @@ void pkd_stop(struct pkd_result *out) {
     int rc = 0;
 
     ctx.keep_going = 0;
+    close(pkd_state.server_fd);
 
     rc = pthread_kill(ctx.tid, SIGUSR1);
     assert_int_equal(rc, 0);