Add support for /usr/etc configurations

Created by: sysrich

An increasing number of distributions are using locations in /usr (such as /usr/etc) for packaging their distribution configuration.

This is to enable /etc to be the location for "system wide" configuration without complicating a distributions ability to ship sensible defaults

A large number of upstreams used by openSUSE have already adopted support for this, including pam, netcfg, openssh, etc https://en.opensuse.org/openSUSE:Packaging_UsrEtc for more info

I'm keen to use xdg-terminal-exec on some projects I have around openSUSE MicroOS, so I'd like this to work here also :)

Kind Regards, Richard