relay: stable configuration IDs

Custom provider cannot 'shadow' the relays from the config.
e.g. lights will use the id=0 when relay control is enabled, and
we should not use relayGpio0, relayPulse0, relayBoot0 and etc.

code/espurna/relay.cpp

@@ -1985,9 +1985,10 @@ RelayProviderBasePtr _relaySetupProvider(unsigned char index) {
 }
 
 void _relaySetup() {
-    _relays.reserve(_relayAdhocPins());
+    auto relays = _relays.size();
+    _relays.reserve(relays + _relayAdhocPins());
 
-    for (unsigned char id = 0; id < RelaysMax; ++id) {
+    for (unsigned char id = relays; id < RelaysMax; ++id) {
         auto impl = _relaySetupProvider(id);
         if (!impl) {
             break;