[RFC] Fix install permissions

This was a bit more work than I'd hoped for, but I think it achieves the goal.  We will now do our best to make sure that Neovim is installed in a useful and not be corrupted by the user's umask.  If directories already exist, those directories are left alone with their existing permissions--just like the Linux install command.

This supersedes #1201 and #1086.
If we add some new filetype to `/runtime/`, we will need to remember to update this, right?