wg-quick invoking resolvectl instead of resolvconf on systems where that is appropriate?

[Arjen Runsink]

In response to messages:

https://lists.zx2c4.com/pipermail/wireguard/2019-September/004524.html
https://lists.zx2c4.com/pipermail/wireguard/2019-September/004533.html

The resolvconf implementation on distro's using systemd is not a 
complete replacement of the original resolvconf.

On my preferred distribution (Solus) it even results in some 
strangeness where instead of setting the dns, resolvconf produces the 
status and halts the execution of wg-quick.

My solution is to detect the presence of systemd during build and use a 
modified version of linux.bash that I call linux-systemd.bash which 
uses resolvectl and a changed Makefile to go with it. It is a bit of a 
hack, but if interested, I will gladly send the patch.

Regards, Arjen