transaction: Add ModuleTransaction type and ModuleHandler interface

This allows to easily define go-handlers for module operations.

We need to expose few more types externally so that it's possible to
create the module transaction handler and return specific transaction
errors

module-transaction.go

@@ -0,0 +1,29 @@
+// Package pam provides a wrapper for the PAM application API.
+package pam
+
+// ModuleTransaction is an interface that a pam module transaction
+// should implement.
+type ModuleTransaction interface {
+	SetItem(Item, string) error
+	GetItem(Item) (string, error)
+	PutEnv(nameVal string) error
+	GetEnv(name string) string
+	GetEnvList() (map[string]string, error)
+}
+
+// ModuleHandlerFunc is a function type used by the ModuleHandler.
+type ModuleHandlerFunc func(ModuleTransaction, Flags, []string) error
+
+// ModuleTransaction is the module-side handle for a PAM transaction.
+type moduleTransaction = transactionBase
+
+// ModuleHandler is an interface for objects that can be used to create
+// PAM modules from go.
+type ModuleHandler interface {
+	AcctMgmt(ModuleTransaction, Flags, []string) error
+	Authenticate(ModuleTransaction, Flags, []string) error
+	ChangeAuthTok(ModuleTransaction, Flags, []string) error
+	CloseSession(ModuleTransaction, Flags, []string) error
+	OpenSession(ModuleTransaction, Flags, []string) error
+	SetCred(ModuleTransaction, Flags, []string) error
+}