Feature: lrmd,liblrmd: add lrmd IPC operations for requesting and acknowledging shutdown

This adds two new lrmd IPC operations, LRMD_IPC_OP_SHUTDOWN_REQ and
LRMD_IPC_OP_SHUTDOWN_ACK, along with functions to send them.
This will support the ability to stop pacemaker_remote gracefully.

At this point, no code uses these new operations.