[proxy] Autoconfigure TLS based on daemon settings

With this on OSX you can:
```sh
$ boot2docker init
$ boot2docker up
$ eval $(boot2docker shellinit)
$ weave launch
$ eval $(weave env)
```

prog/docker_tls_args/main.go

@@ -0,0 +1,77 @@
+// docker_tls_args: find the docker daemon's tls args
+// This reimplements pgrep, because pgrep will only look in /proc, but not in
+// $PROCFS.
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+)
+
+func main() {
+	procRoot := os.Getenv("PROCFS")
+	if procRoot == "" {
+		procRoot = "/proc"
+	}
+
+	dirEntries, err := ioutil.ReadDir(procRoot)
+	checkErr(err)
+
+	for _, dirEntry := range dirEntries {
+		dirName := dirEntry.Name()
+		if _, err := strconv.Atoi(dirName); err != nil {
+			continue
+		}
+
+		if comm, err := ioutil.ReadFile(filepath.Join(procRoot, dirName, "comm")); err != nil || string(comm) != "docker\n" {
+			continue
+		}
+
+		cmdline, err := ioutil.ReadFile(filepath.Join(procRoot, dirName, "cmdline"))
+		if err != nil {
+			continue
+		}
+
+		isDaemon := false
+		tlsArgs := []string{}
+		args := bytes.Split(cmdline, []byte{'\000'})
+		for i := 0; i < len(args); i++ {
+			arg := string(args[i])
+			switch {
+			case arg == "-d":
+				isDaemon = true
+				break
+			case arg == "--tls", arg == "--tlsverify":
+				tlsArgs = append(tlsArgs, arg)
+			case strings.HasPrefix(arg, "--tls"):
+				tlsArgs = append(tlsArgs, arg)
+				if len(args) > i+1 &&
+					!strings.Contains(arg, "=") &&
+					!strings.HasPrefix(string(args[i+1]), "-") {
+					tlsArgs = append(tlsArgs, string(args[i+1]))
+					i++
+				}
+			}
+		}
+		if !isDaemon {
+			continue
+		}
+
+		fmt.Println(strings.Join(tlsArgs, " "))
+		os.Exit(0)
+	}
+
+	os.Exit(1)
+}
+
+func checkErr(err error) {
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}

prog/weaveexec/Dockerfile

@@ -18,6 +18,6 @@ RUN apk add --update \
   && rm -rf /var/cache/apk/*
 
 ADD ./weave ./sigproxy ./weaveproxy /home/weave/
-ADD ./netcheck ./weavehosts /usr/bin/
+ADD ./netcheck ./weavehosts ./docker_tls_args /usr/bin/
 ADD ./weavewait /w/w
 ADD ./docker.tgz /