pkg/reexec: move reexec code to a new package

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)

reexec/MAINTAINERS

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+Michael Crosby <michael@docker.com> (@crosbymichael)

reexec/README.md

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+## reexec
+
+The `reexec` package facilitates the busybox style reexec of the docker binary that we require because 
+of the forking limitations of using Go.  Handlers can be registered with a name and the argv 0 of 
+the exec of the binary will be used to find and execute custom init paths.

reexec/reexec.go

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+package reexec
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+var registeredInitializers = make(map[string]func())
+
+// Register adds an initialization func under the specified name
+func Register(name string, initializer func()) {
+	if _, exists := registeredInitializers[name]; exists {
+		panic(fmt.Sprintf("reexec func already registred under name %q", name))
+	}
+
+	registeredInitializers[name] = initializer
+}
+
+// Init is called as the first part of the exec process and returns true if an
+// initialization function was called.
+func Init() bool {
+	initializer, exists := registeredInitializers[os.Args[0]]
+	if exists {
+		initializer()
+
+		return true
+	}
+
+	return false
+}
+
+// Self returns the path to the current processes binary
+func Self() string {
+	name := os.Args[0]
+
+	if filepath.Base(name) == name {
+		if lp, err := exec.LookPath(name); err == nil {
+			name = lp
+		}
+	}
+
+	return name
+}