RPC: Limit generate command to regtest
RPC: Limit generate command to regtest

It doesn't make sense on other networks; closes #191

nlgcoin/guldencoin-official,nlgcoin/guldencoin-official,nlgcoin/guldencoin-official,nlgcoin/guldencoin-official,nlgcoin/guldencoin-official,nlgcoin/guldencoin-official