Add setuid & setgid options

Only applicable on unix systems, although Go doesn't support Linux at
this time.

droppriv_unix.go

@@ -0,0 +1,21 @@
+// +build !windows,!linux
+
+package main
+
+import (
+	"syscall"
+)
+
+func dropPrivileges(uid, gid int) error {
+	err := syscall.Setgid(gid)
+	if err != nil {
+		return err
+	}
+
+	err = syscall.Setuid(uid)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}