module-transaction: Do not allow parallel conversations by default

Pam conversations per se may also run in parallel, but this implies that
the application supports this.

Since this normally not the case, do not create modules that may invoke
the pam conversations in parallel by default, adding a mutex to protect
such calls.

cmd/pam-moduler/tests/integration-tester-module/pam_module.go

@@ -1,4 +1,4 @@
-// Code generated by "pam-moduler -type integrationTesterModule"; DO NOT EDIT.
+// Code generated by "pam-moduler -type integrationTesterModule -parallel-conv"; DO NOT EDIT.
 
 //go:generate go build "-ldflags=-extldflags -Wl,-soname,pam_go.so" -buildmode=c-shared -o pam_go.so -tags go_pam_module
 
@@ -43,7 +43,7 @@ func handlePamCall(pamh *C.pam_handle_t, flags C.int, argc C.int,
 		return C.int(pam.ErrIgnore)
 	}
 
-	mt := pam.NewModuleTransactionInvoker(pam.NativeHandle(pamh))
+	mt := pam.NewModuleTransactionInvokerParallelConv(pam.NativeHandle(pamh))
 	err := mt.InvokeHandler(moduleFunc, pam.Flags(flags),
 		sliceFromArgv(argc, argv))
 	if err == nil {