engine: resources: Add a tftp server and tftp file resource

This adds a tftp server and tftp file resource to help you run a small
pure golang tftp server embedded inside the mgmt resource model.

examples/lang/tftp0.mcl

@@ -0,0 +1,44 @@
+$root = "/tmp/tftproot/"
+file $root {
+	state => "exists",
+}
+file "${root}file0" {
+	content => "i'm file0 in ${root}\n",
+	state => "exists",
+}
+
+tftp:server ":1069" {	# by default tftp uses :69 but using :1069 avoids needing root!
+	#address => ":1069",	# you can override the name like this
+	timeout => 60,		# increase the timeout
+	root => $root,		# add a tftproot (optional)
+}
+
+# you can add a raw file like this...
+tftp:file "/file1" {
+	data => "hello, world, i'm file1 and i don't exist on disk!\n",
+}
+
+# or if there's a file on disk you care about...
+$f2 = "/tmp/some_file"
+file $f2 {
+	content => "i'm a cool file in /tmp\n",
+	state => "exists",
+}
+
+# you can point to it directly...
+tftp:file "/file2" {
+	path => $f2,
+
+	Depend => File[$f2],	# TODO: add autoedges
+}
+
+# here's a file in the middle of nowhere that still works...
+tftp:file "/i/am/some/deeply/nested/file" {
+	data => "how did you find me!\n",
+}
+
+# and this file won't autogroup with the main tftp server
+tftp:file "/nope/noway" {
+	data => "i won't be seen!\n",
+	server => "someone else!",	# normally we don't use this this way
+}

examples/tftp/tftp_client.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/pin/tftp"
+)
+
+func main() {
+	if len(os.Args) < 2 || len(os.Args) > 3 {
+		log.Printf("Usage: ./%s [port] <filename>", os.Args[0])
+		return
+	}
+
+	port := "69" // default
+	if len(os.Args) >= 3 {
+		port = os.Args[1]
+	}
+	file := os.Args[len(os.Args)-1]
+
+	addr := fmt.Sprintf("127.0.0.1:%s", port)
+	log.Printf("Connecting to: %s", addr)
+
+	c, err := tftp.NewClient(addr)
+	if err != nil {
+		log.Printf("Error connecting to server: %v", err)
+		return
+	}
+	wt, err := c.Receive(file, "octet") // no idea why this is "octet"
+	if err != nil {
+		log.Printf("Error receiving from server: %v", err)
+		return
+	}
+
+	// Optionally obtain transfer size before actual data.
+	if n, ok := wt.(tftp.IncomingTransfer).Size(); ok {
+		log.Printf("Transfer size: %d", n)
+	}
+
+	buf := new(bytes.Buffer)
+	n, err := wt.WriteTo(buf)
+
+	log.Printf("%d bytes received", n)
+	log.Printf("Got: %s", buf.String())
+
+	log.Printf("Done!")
+}