Add setuid & setgid options

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

droppriv_nope.go

@@ -0,0 +1,12 @@
+// +build linux windows
+
+package main
+
+import (
+	"errors"
+	"runtime"
+)
+
+func dropPrivileges(uid, gid int) error {
+	return errors.New("setuid and setgid not supported on " + runtime.GOOS)
+}