Switch from NetworkManager to dhclient

NetworkManager is a memory hog.  We don't need it to do anything except
renew the DHCP lease when it's time, so let's switch to a more
lightweight alternative.  Unfortunately, Fedora does not include any
systemd units for `dhclient`, so we have to provide our own.

overlay/etc/systemd/system/dhclient@.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=DHCPv4 client for %I
+Before=network-online.target
+Before=network.target
+
+[Service]
+Type=forking
+RuntimeDirectory=dhclient
+ExecStart=/usr/sbin/dhclient -4 -1 -lf /run/dhclient/%I.lease -pf /run/dhclient/%I.pid
+PIDFile=/run/dhclient/%I.pid
+
+[Install]
+WantedBy=multi-user.target