lib: posix: sleep() should report unslept time in seconds

In the case that `sleep()` is interrupted, the POSIX spec requires
it to return the number of "unslept" seconds (i.e. the number of
seconds requested minus the number of seconds actually slept).

Since `k_sleep()` already returns the amount of "unslept" time
in ms, we can simply use that.

Signed-off-by: Chris Friedt <cfriedt@meta.com>

lib/posix/sleep.c

@@ -14,8 +14,12 @@
  */
 unsigned sleep(unsigned int seconds)
 {
-	k_sleep(K_SECONDS(seconds));
-	return 0;
+	int rem;
+
+	rem = k_sleep(K_SECONDS(seconds));
+	__ASSERT_NO_MSG(rem >= 0);
+
+	return rem / MSEC_PER_SEC;
 }
 /**
  * @brief Suspend execution for microsecond intervals.