Recursive config parsing seems not to ignore backup files

I'm getting e.g.:

```
WARNING:pkgcore:'/etc/portage/package.use/local.conf~', line 2: parsing error: token -nginx_modules_http_* is not a valid use flag
```

I'm guessing this means that pkgcore doesn't ignore files ending with `~` like Portage does. Yes, I know Portage config support is a mess, sigh.