IN  ULONG            PostFlags,
  IN  PCM_KEY_BODY     KeyBody,
  IN  PCM_POST_BLOCK   MasterBlock

Routine Description:

    Allocates a post block from pool.  The non-pageable stuff comes from
    NonPagedPool, the pageable stuff from paged pool.  Quota will be


    BlockType  - specifies the type of the post block to be allocated
                i.e. : PostSyncrhronous, PostAsyncUser, PostAsyncKernel

    PostFlags      - specifies the flags to be set on the allocated post block
                valid flags:
                    - REG_NOTIFY_MASTER_POST - the post block to be allocated
                      is a master post block.
    KeyBody     - The Key object to whom this post block is attached. On master blocks
                  this is NULL. When the post object is freed, the KeyBody object is
                  dereferenced (if not NULL - i.e. for slave blocks). This allow us to
                  perform back-end cleanup for "fake-slave" keys opened by NtNotifyMultipleKeys
    MasterBlock - the post block to be allocated is a slave of this master block.
                  valid only when PostFlags ==  REG_NOTIFY_MASTER_POST

Obs: The Sync.SystemEvent and AsyncUser.Apc members are allocated only for master post blocks

Return Value:

    Pointer to the CM_POST_BLOCK if successful

    NULL if there were not enough resources available.