transaction, moduler: Do not export PAM conv handler function to modules

This function is only needed when using go PAM for creating applications
so it's not something we expect to have exported to library modules.

To prevent this use an `asPamModule` tag to prevent compilation of
application-only features.

app-transaction.go

@@ -0,0 +1,24 @@
+//go:build !go_pam_module
+
+package pam
+
+/*
+#include <security/pam_appl.h>
+#include <stdint.h>
+*/
+import "C"
+
+import "runtime/cgo"
+
+// _go_pam_conv_handler is a C wrapper for the conversation callback function.
+//
+//export _go_pam_conv_handler
+func _go_pam_conv_handler(msg *C.struct_pam_message, c C.uintptr_t, outMsg **C.char) C.int {
+	convHandler, ok := cgo.Handle(c).Value().(ConversationHandler)
+	if !ok || convHandler == nil {
+		return C.int(ErrConv)
+	}
+	replyMsg, r := pamConvHandler(Style(msg.msg_style), msg.msg, convHandler)
+	*outMsg = replyMsg
+	return r
+}