Update launcher script to download and call launcher rewrite

download, checksum and use https://github.com/discourse/launcher

support update arg to re-download launcher binary.

keep most of the prereqs function in place.

check for autocompletion and squelch output if we're calling autocomplete.

Remove checks on config existence and config permissions. If we need this check
it should go into the launcher app.

update overlay check to include overlayfs which is what comes up on default latest
docker on desktop versions

Author: featheredtoast