include/ipc: Add method for unsubscribing an endpoint from topics

include/ipc: Add method for unsubscribing an endpoint from topics

The IPC module does not provide a function for unsubscribing an
endpoint from a pubsub topic, forcing a script that wishes to
unsubscribe from a topic to close the endpoint and open a new one.

This commit adds the `ipc_endpoint_unsubscribe()' method to the IPC
module, allowing an endpoint to be unsubscribed from pubsub topics.