If users wrote a FQDN in ipv4.dhcp-hostname presumably it's because
they really want to send the full value, not only the host part, so
let's send it as-is.

This obviously is a change in behavior, but only for users that have a
FQDN in ipv4.dhcp-hostname, where it's not clear if they really want the
domain to be stripped.

When the property is unset, we keep sending only the host part of the
system hostname to maintain backwards compatibility.

This commit aligns NM behavior to initscripts.

(cherry picked from commit cf5fab8f)