pam-moduler: Add first implementation of a Go PAM Module generator

A PAM module can be generated using pam-moduler and implemented fully in
go without having to manually deal with the C setup.

Module can be compiled using go generate, so go:generate directives can be
used to make this process automatic, with a single go generate call as shown
in the example.

module-transaction.go

@@ -15,7 +15,9 @@ type ModuleTransaction interface {
 type ModuleHandlerFunc func(ModuleTransaction, Flags, []string) error
 
 // ModuleTransaction is the module-side handle for a PAM transaction.
-type moduleTransaction = transactionBase
+type moduleTransaction struct {
+	transactionBase
+}
 
 // ModuleHandler is an interface for objects that can be used to create
 // PAM modules from go.
@@ -27,3 +29,9 @@ type ModuleHandler interface {
 	OpenSession(ModuleTransaction, Flags, []string) error
 	SetCred(ModuleTransaction, Flags, []string) error
 }
+
+// NewModuleTransaction allows initializing a transaction invoker from
+// the module side.
+func NewModuleTransaction(handle NativeHandle) ModuleTransaction {
+	return &moduleTransaction{transactionBase{handle: handle}}
+}