Install a script named foo; it will go inside a directory named "foo" inside gm_scripts. Reinstall it. It will go to "foo-1". After install, the old copy is uninstalled. A second reinstall will thus go to "foo", and repeat. Ideally re-installing the same script would keep the same install location.
A related problem: reinstall a script, then go to the Add-ons Manager, right-click the script name and select Edit. The script can't be loaded, because it's still looking for it in the previous directory.
It works OK after Firefox is restarted.
Do GM_config.install() _before_ putting a RemoteScript's files in place.
This cleans the old script's files out first, so the names on disk do not collide.
Fixes: #1515, #1520