Previouly, there are three channels for multi-process
(i.e., primary/secondary) communication.
  1. Config-file based channel, in which, the primary process writes
     info into a pre-defined config file, and the secondary process
     reads the info out.
  2. vfio submodule has its own channel based on unix socket for the
     secondary process to get container fd and group fd from the
     primary process.
  3. pdump submodule also has its own channel based on unix socket for
     packet dump.

It'd be good to have a generic communication channel for multi-process
communication to accomodate the requirements including:
  a. Secondary wants to send info to primary, for example, secondary
     would like to send request (about some specific vdev to primary).
  b. Sending info at any time, instead of just initialization time.
  c. Share FDs with the other side, for vdev like vhost, related FDs
     (memory region, kick) should be shared.
  d. A send message request needs the other side to response immediately.

This patch proposes to create a communication channel, based on datagram
unix socket, for above requirements. Each process will block on a unix
socket waiting for messages from the peers.

Three new APIs are added:

  1. rte_eal_mp_action_register() is used to register an action,
     indexed by a string, when a component at receiver side would like
     to response the messages from the peer processe.
  2. rte_eal_mp_action_unregister() is used to unregister the action
     if the calling component does not want to response the messages.
  3. rte_eal_mp_sendmsg() is used to send a message, and returns
     immediately. If there are n secondary processes, the primary
     process will send n messages.

Suggested-by: Konstantin Ananyev <redacted>
Signed-off-by: Jianfeng Tan <redacted>
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Acked-by: Konstantin Ananyev <redacted>