Add static_build tag which avoids user.Current()

Its not safe to use os/user.Current() in a statically linked program.
You will be getting warnings like:

 warning: Using 'getpwnam_r' in statically linked applications requires
  at runtime the shared libraries from the glibc version used for linking
 warning: Using 'getpwuid_r' in statically linked applications requires
  at runtime the shared libraries from the glibc version used for linking

We avoid this when building with -tag static_build by parsing
/etc/passwd directly for the homedir.

auth.go

@@ -6,7 +6,6 @@ import (
 	"errors"
 	"io"
 	"os"
-	"os/user"
 	"strconv"
 )
 
@@ -55,11 +54,7 @@ type Auth interface {
 func (conn *Conn) Auth(methods []Auth) error {
 	if methods == nil {
 		uid := strconv.Itoa(os.Getuid())
-		u, err := user.Current()
-		if err != nil {
-			return err
-		}
-		methods = []Auth{AuthExternal(uid), AuthCookieSha1(uid, u.HomeDir)}
+		methods = []Auth{AuthExternal(uid), AuthCookieSha1(uid, getHomeDir())}
 	}
 	in := bufio.NewReader(conn.transport)
 	err := conn.transport.SendNullByte()

homedir.go

@@ -0,0 +1,28 @@
+package dbus
+
+import (
+	"os"
+	"sync"
+)
+
+var (
+	homeDir     string
+	homeDirLock sync.Mutex
+)
+
+func getHomeDir() string {
+	homeDirLock.Lock()
+	defer homeDirLock.Unlock()
+
+	if homeDir != "" {
+		return homeDir
+	}
+
+	homeDir = os.Getenv("HOME")
+	if homeDir != "" {
+		return homeDir
+	}
+
+	homeDir = lookupHomeDir()
+	return homeDir
+}

homedir_dynamic.go

@@ -0,0 +1,15 @@
+// +build !static_build
+
+package dbus
+
+import (
+	"os/user"
+)
+
+func lookupHomeDir() string {
+	u, err := user.Current()
+	if err != nil {
+		return "/"
+	}
+	return u.HomeDir
+}

homedir_static.go

@@ -0,0 +1,45 @@
+// +build static_build
+
+package dbus
+
+import (
+	"bufio"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func lookupHomeDir() string {
+	myUid := os.Getuid()
+
+	f, err := os.Open("/etc/passwd")
+	if err != nil {
+		return "/"
+	}
+	defer f.Close()
+
+	s := bufio.NewScanner(f)
+
+	for s.Scan() {
+		if err := s.Err(); err != nil {
+			break
+		}
+
+		line := strings.TrimSpace(s.Text())
+		if line == "" {
+			continue
+		}
+
+		parts := strings.Split(line, ":")
+
+		if len(parts) >= 6 {
+			uid, err := strconv.Atoi(parts[2])
+			if err == nil && uid == myUid {
+				return parts[5]
+			}
+		}
+	}
+
+	// Default to / if we can't get a better value
+	return "/"
+}