install: Add ability to append network config

* If a network tail file is found, append to the generated config.
* This functionality allows users to create a tail file @
  /etc/systemd/network/template/dosync-${interface}.network.tail to
  add additional options to the network config.
* Example use case: Adding "Tunnel=name-of-tunnel" to the eth0
  interface file to signal systemd-networkd to bring up a tunnel using
  the eth0 interface.

install.sh

@@ -912,6 +912,11 @@ process_interface() {
 			fi
 			log "Added IPv6 address ${address}/${prefix} on ${interface}."
 		fi
+		local network_tail=/etc/systemd/network/template/dosync-${interface}.network.tail
+		if [[ -r "${network_tail}" ]]; then
+			cat ${network_tail}
+			log "Appended user specified config for ${interface}."
+		fi
 	} > /run/systemd/network/dosync-${interface}.network
 }