Reviewed and added OSX patch

The patch was provided by:
Naveen Nathan <naveen@lastninja.net>

The following modifications to the patch was made:

- Added copyright notice
- Fixed file descriptor leak in .MTU() method
- Migrated to the new(er) golang.org/x/sys/unix package
- Removed non-functioning Daemonize method

src/daemon_darwin.go

@@ -0,0 +1,9 @@
+package main
+
+import (
+	"errors"
+)
+
+func Daemonize() error {
+	return errors.New("Not implemented on OSX")
+}