Only enable mainnet RPC method safety check on mainnet.

This fixes the unsafemainnet option so it only affects mainnet.  This
broke as part of the recent btcwallet sync as the entire feature had
to be rewritten for the new legacyrpc package.

Fixes #85.

rpc/legacyrpc/methods.go

@@ -221,7 +221,8 @@ type lazyHandler func() (interface{}, *dcrjson.RPCError)
 // chainClient is not nil, the returned handler performs RPC passthrough.
 func lazyApplyHandler(request *dcrjson.Request, w *wallet.Wallet, chainClient *chain.RPCClient, unsafeMainNet bool) lazyHandler {
 	handlerData, ok := rpcHandlers[request.Method]
-	if ok && handlerData.requireUnsafeOnMainNet && !unsafeMainNet {
+	if ok && handlerData.requireUnsafeOnMainNet &&
+		w.ChainParams() == &chaincfg.MainNetParams && !unsafeMainNet {
 		return func() (interface{}, *dcrjson.RPCError) {
 			return nil, &ErrMainNetSafety
 		}