Add pledge(2) support for OpenBSD (gopasspw#469)

Pledge (OpenBSD only) is a way to restrict the operations an
application can make use of on OpenBSD. Applications declare
their intent by specifying the system calls they will use,
after which point, any use outside of the pledge'd calls will
result in the kernel killing the process.

More info on pledge can be found here: http://man.openbsd.org/pledge

main.go

@@ -20,6 +20,7 @@ import (
 	"github.com/justwatchcom/gopass/store/sub"
 	"github.com/justwatchcom/gopass/utils/ctxutil"
 	"github.com/justwatchcom/gopass/utils/out"
+	"github.com/justwatchcom/gopass/utils/protect"
 	colorable "github.com/mattn/go-colorable"
 	"github.com/urfave/cli"
 )
@@ -38,6 +39,7 @@ var (
 )
 
 func main() {
+	protect.Pledge("stdio rpath tty proc exec")
 	ctx := context.Background()
 
 	// trap Ctrl+C and call cancel on the context

utils/protect/protect.go

@@ -0,0 +1,8 @@
+// +build !openbsd
+
+package protect
+
+// Pledge on any other system than OpenBSD doesn't do anything
+func Pledge(s string) {
+	return
+}

utils/protect/protect_openbsd.go

@@ -0,0 +1,11 @@
+// +build openbsd
+
+package protect
+
+import "golang.org/x/sys/unix"
+
+// Pledge on OpenBSD lets us "promise" to only run a subset of
+// system calls: http://man.openbsd.org/pledge
+func Pledge(s string) {
+	_ = unix.Pledge(s, nil)
+}