Make CoreDNS a server type plugin for Caddy (#220)

* Make CoreDNS a server type plugin for Caddy

Remove code we don't need and port all middleware over. Fix all tests
and rework the documentation.

Also make `go generate` build a caddy binary which we then copy into
our directory. This means `go build`-builds remain working as-is.

And new etc instances in each etcd test for better isolation.
Fix more tests and rework test.Server with the newer support Caddy offers.

Fix Makefile to support new mode of operation.

middleware/rewrite/setup.go

@@ -0,0 +1,115 @@
+package rewrite
+
+import (
+	"strconv"
+	"strings"
+
+	"github.com/miekg/coredns/core/dnsserver"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("rewrite", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	rewrites, err := rewriteParse(c)
+	if err != nil {
+		return err
+	}
+
+	dnsserver.GetConfig(c).AddMiddleware(func(next dnsserver.Handler) dnsserver.Handler {
+		return Rewrite{Next: next, Rules: rewrites}
+	})
+
+	return nil
+}
+
+func rewriteParse(c *caddy.Controller) ([]Rule, error) {
+	var simpleRules []Rule
+	var regexpRules []Rule
+
+	for c.Next() {
+		var rule Rule
+		var err error
+		var base = "."
+		var pattern, to string
+		var status int
+		var ext []string
+
+		args := c.RemainingArgs()
+
+		var ifs []If
+
+		switch len(args) {
+		case 1:
+			base = args[0]
+			fallthrough
+		case 0:
+			for c.NextBlock() {
+				switch c.Val() {
+				case "r", "regexp":
+					if !c.NextArg() {
+						return nil, c.ArgErr()
+					}
+					pattern = c.Val()
+				case "to":
+					args1 := c.RemainingArgs()
+					if len(args1) == 0 {
+						return nil, c.ArgErr()
+					}
+					to = strings.Join(args1, " ")
+				case "ext": // TODO(miek): fix or remove
+					args1 := c.RemainingArgs()
+					if len(args1) == 0 {
+						return nil, c.ArgErr()
+					}
+					ext = args1
+				case "if":
+					args1 := c.RemainingArgs()
+					if len(args1) != 3 {
+						return nil, c.ArgErr()
+					}
+					ifCond, err := NewIf(args1[0], args1[1], args1[2])
+					if err != nil {
+						return nil, err
+					}
+					ifs = append(ifs, ifCond)
+				case "status": // TODO(miek): fix or remove
+					if !c.NextArg() {
+						return nil, c.ArgErr()
+					}
+					status, _ = strconv.Atoi(c.Val())
+					if status < 200 || (status > 299 && status < 400) || status > 499 {
+						return nil, c.Err("status must be 2xx or 4xx")
+					}
+				default:
+					return nil, c.ArgErr()
+				}
+			}
+			// ensure to or status is specified
+			if to == "" && status == 0 {
+				return nil, c.ArgErr()
+			}
+			// TODO(miek): complex rules
+			base, pattern, to, status, ext, ifs = base, pattern, to, status, ext, ifs
+			err = err
+			// if rule, err = NewComplexRule(base, pattern, to, status, ext, ifs); err != nil {
+			// return nil, err
+			// }
+			regexpRules = append(regexpRules, rule)
+
+		// the only unhandled case is 2 and above
+		default:
+			rule = NewSimpleRule(args[0], strings.Join(args[1:], " "))
+			simpleRules = append(simpleRules, rule)
+		}
+	}
+
+	// put simple rules in front to avoid regexp computation for them
+	return append(simpleRules, regexpRules...), nil
+}