lang: core: util: Add hostname mapper function

This adds a new util package with some useful functionality which could
be implemented as pure mcl, but instead we add it here as a good place
to help with code reuse.

lang/core/core.go

@@ -55,6 +55,7 @@ import (
 	_ "github.com/purpleidea/mgmt/lang/core/strings"
 	_ "github.com/purpleidea/mgmt/lang/core/sys"
 	_ "github.com/purpleidea/mgmt/lang/core/test"
+	_ "github.com/purpleidea/mgmt/lang/core/util"
 	_ "github.com/purpleidea/mgmt/lang/core/value"
 	_ "github.com/purpleidea/mgmt/lang/core/world"
 )