Previously, the generated config file would contain arbitrary whitespace that
made it look ugly.
* gnu/services/vpn.scm (<wireguard-configuration>) [dns]: Change default value
from #f to '().
(wireguard-configuration-file): Use match-record. Format each line
individually, assembling the lines at the end to avoid extraneous white space.
* doc/guix.texi (VPN Services): Update doc.
