bar: wait for all modules to have started up before continuing

After starting all the module threads, wait for all modules to have
signalled "ready" before continuing.

This will allow modules to do initial setup, and knowing that
content() will *not* be called until they've signalled "ready".

modules/clock/clock.c

@@ -1,6 +1,8 @@
 #include "clock.h"
 #include <stdlib.h>
+#include <stdint.h>
 #include <time.h>
+#include <unistd.h>
 #include <assert.h>
 
 #include <poll.h>
@@ -50,6 +52,8 @@ run(struct module_run_context *ctx)
 {
     const struct bar *bar = ctx->module->bar;
 
+    write(ctx->ready_fd, &(uint64_t){1}, sizeof(uint64_t));
+
     while (true) {
         time_t now = time(NULL);
         time_t now_no_secs = now / 60 * 60;