Add autoload for host manipulation
test: Fix restart() in test/functional/helpers.lua

Always redefine the cleanup function so it can be used as a test hook
runtime: Allow a channel id to be passed to rpc#host#Register

This simplifies testing functions that are supposed to bootstrap a host
runtime: Add autoload/rpc/define.vim and test

This new vimscript module will contain common code used by hosts that need to
define commands, autocommands and functions to be executed over msgpack-rpc.