Add PAM module stub (build deferred until libpam0g-dev installed)

pam/pam_ubuntu_fido_pending.c

@@ -0,0 +1,18 @@
+#define PAM_SM_AUTH
+#include <security/pam_modules.h>
+#include <security/pam_ext.h>
+#include <syslog.h>
+#include <stdio.h>
+
+PAM_EXTERN int pam_sm_authenticate(pam_handle_t *pamh, int flags,
+                                    int argc, const char **argv) {
+    (void)flags; (void)argc; (void)argv;
+    pam_syslog(pamh, LOG_INFO, "ubuntu_fido_pending: stub - allowing");
+    return PAM_IGNORE; /* implemented in Phase 6 */
+}
+
+PAM_EXTERN int pam_sm_setcred(pam_handle_t *pamh, int flags,
+                              int argc, const char **argv) {
+    (void)pamh; (void)flags; (void)argc; (void)argv;
+    return PAM_SUCCESS;
+}