Posix: use /etc/dub/settings.json if DUB is installed in /usr

For Linux distributions that put the dub installation in /usr, there is now a
special case that DUB will load from /etc/dub/settings.json (absolute path) if
the installation is inside /usr.

Previously settings would have attempted to be loaded from
`/usr/etc/dub/settings.json` if installed in `/usr/bin/dub`. This is still
loaded if it exists, but if not `/etc/dub/settings.json` will be loaded.