runtime: Refer to plugins running outside Nvim as "remote plugins"

This is @elmart's suggestion. Remote is a better term than rpc or external. 

I'm tempted to also rename the msgpack-rpc functions from `rpcrequest`/`rpcnotify`/`rpcstart`/`rpcstop` to `rrequest`/`rnotify`/`rstart`/`rstop`, hiding protocol details from the public API. What do you guys think?