PG守护进程（Postmaster）——建立共享内存和信号量reset_shared

reset_shared函数重置共享内存和信号量，使用port作为IPC键。如果使用SysV shmem and/or semas，在每个cycle of life中使用相同的IPC键。

/* reset_shared -- reset shared memory and semaphores */
static void reset_shared(int port) {
  /* Create or re-create shared memory and semaphores.
   * Note: in each "cycle of life" we will normally assign the same IPC keys
   * (if using SysV shmem and/or semas), since the port number is used to
   * determine IPC keys.  This helps ensure that we will clean up dead IPC
   * objects if the postmaster crashes and is restarted. */
  CreateSharedMemoryAndSemaphores(port);
}