Fix relative path execution of docker daemon in reexec.Self()

After the new libcontainer API, the reexec.Self() output of the daemon
binary is used as the libcontainer factory InitPath.  If it is relative,
it can't be found at container start time.  This patch solves the
problem by making sure that we return a rooted/absolute path if a
relative path is used.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

reexec/reexec.go

@@ -35,8 +35,14 @@ func Self() string {
 	name := os.Args[0]
 	if filepath.Base(name) == name {
 		if lp, err := exec.LookPath(name); err == nil {
-			name = lp
+			return lp
 		}
 	}
+	// handle conversion of relative paths to absolute
+	if absName, err := filepath.Abs(name); err == nil {
+		return absName
+	}
+	// if we coudn't get absolute name, return original
+	// (NOTE: Go only errors on Abs() if os.Getwd fails)
 	return name
 }