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.

example-module/module.go

@@ -0,0 +1,50 @@
+// These go:generate directive allow to generate the module by just using
+// `go generate` once in the module directory.
+// This is not strictly needed
+
+//go:generate go run github.com/msteinert/pam/v2/cmd/pam-moduler
+//go:generate go generate --skip="pam_module.go"
+
+// Package main provides the module shared library.
+package main
+
+import (
+	"fmt"
+
+	"github.com/msteinert/pam/v2"
+)
+
+type exampleHandler struct{}
+
+var pamModuleHandler pam.ModuleHandler = &exampleHandler{}
+var _ = pamModuleHandler
+
+// AcctMgmt is the module handle function for account management.
+func (h *exampleHandler) AcctMgmt(mt pam.ModuleTransaction, flags pam.Flags, args []string) error {
+	return fmt.Errorf("AcctMgmt not implemented: %w", pam.ErrIgnore)
+}
+
+// Authenticate is the module handle function for authentication.
+func (h *exampleHandler) Authenticate(mt pam.ModuleTransaction, flags pam.Flags, args []string) error {
+	return pam.ErrAuthinfoUnavail
+}
+
+// ChangeAuthTok is the module handle function for changing authentication token.
+func (h *exampleHandler) ChangeAuthTok(mt pam.ModuleTransaction, flags pam.Flags, args []string) error {
+	return fmt.Errorf("ChangeAuthTok not implemented: %w", pam.ErrIgnore)
+}
+
+// OpenSession is the module handle function for open session.
+func (h *exampleHandler) OpenSession(mt pam.ModuleTransaction, flags pam.Flags, args []string) error {
+	return fmt.Errorf("OpenSession not implemented: %w", pam.ErrIgnore)
+}
+
+// CloseSession is the module handle function for close session.
+func (h *exampleHandler) CloseSession(mt pam.ModuleTransaction, flags pam.Flags, args []string) error {
+	return fmt.Errorf("CloseSession not implemented: %w", pam.ErrIgnore)
+}
+
+// SetCred is the module handle function for set credentials.
+func (h *exampleHandler) SetCred(mt pam.ModuleTransaction, flags pam.Flags, args []string) error {
+	return fmt.Errorf("SetCred not implemented: %w", pam.ErrIgnore)
+}