Hey Guix,

This patch updates libexec/dhclient-script to use resolvconf instead of
directly editing /etc/resolv.conf, similar to dhcpcd and friends.

This allows dhclient to play nicely with other clients which may want to
contend for /etc/resolv.conf control. My particular use case is running a local
dnsmasq instance, configured to allow using the various DNS servers of multiple
simultaneous VPN connections.

Ideally, this would be an upstream patch, but ISC announced end of maintenance
in 2022, so until we migrate dhcp-client-service-type to something else, I
believe it makes sense to have the patch here.

Using a bare-bones.scm VM, I have confirmed that this transparently works in
the default case where no /etc/resolvconf.conf is setup. In a separate patch, I
will be sharing my resolvconf-service-type definition that enables the dnsmasq
integration I mention.

Note, however, that switching to this via a system reconfigure has the
possibility of nuking DNS until the next time dhclient tries to update them.
Simply restarting the networking service is enough to force this, though.

Thoughts?