kernel: instant timeout if local target is unreachable

[dr-frmr]

Problem

Addresses #211. The kernel drops messages that cannot be routed locally for one reason or another -- mostly if the process doesn't exist or the sender doesn't have the capability to message the target.

Solution

This PR adds a function that gets called on definitively dropped messages which sends a Timeout error to the sender if the messages was a request and expect(ed) a response. Note that the Timeout would be sent eventually before this, but now it is sent immediately. This lets processes react faster to issues and should generally be done everywhere.

Docs Update

N/A

Notes