lang: interfaces, funcs: Implement fmt.Stringer for functions

This adds the requirement that all function implementations provider a
String() string method so that these can be used as vertices in the
pgraph library. If we eventually move to generics for the pgraph DAG,
then this might not matter, but it's not bad that these have names
either.

lang/interfaces/func.go

@@ -18,6 +18,7 @@
 package interfaces
 
 import (
+	"fmt"
 	"strings"
 
 	"github.com/purpleidea/mgmt/engine"
@@ -57,6 +58,8 @@ type Init struct {
 // TODO: should we support a static version of this interface for funcs that
 // never change to avoid the overhead of the goroutine and channel listener?
 type Func interface {
+	fmt.Stringer // so that this can be stored as a Vertex
+
 	Validate() error // FIXME: this is only needed for PolyFunc. Get it moved and used!
 
 	// Info returns some information about the function in question, which