Allow waiting for debugger.

If the service is started with the single argument "debug" eg by setting
the Start parameters of the service in services.msc, we delayed startup
in service_main() until the await_debugger variable is set to false.

Attach to nssm.exe with your debugger of choice, set a breakpoint within
that function and poke the variable when ready.

Beware that the behaviour of the service manager when service_main
doesn't complete operation promptly is undefined.

service.cpp

@@ -1503,6 +1503,9 @@ void WINAPI service_main(unsigned long argc, TCHAR **argv) {
   nssm_service_t *service = alloc_nssm_service();
   if (! service) return;
 
+  static volatile bool await_debugger = (argc > 1 && str_equiv(argv[1], _T("debug")));
+  while (await_debugger) Sleep(1000);
+
   if (_sntprintf_s(service->name, _countof(service->name), _TRUNCATE, _T("%s"), argv[0]) < 0) {
     log_event(EVENTLOG_ERROR_TYPE, NSSM_EVENT_OUT_OF_MEMORY, _T("service->name"), _T("service_main()"), 0);
     return;