Use single cert/key for RPC by default.
Use single cert/key for RPC by default.

Also, while here, set the default RPC cert/key in the initial config
struct so the default will show when displaying the usage.

jcmincke/dcrd,soapboxsys/ombfullnode,dajohi/btcd,decred/dcrd,dan-da/btcd,jrick/dcrd,1d4Nf6/btcd,ltcsuite/ltcd,stevenroose/btcd,btcsuitereleases/btcd,toddfries/btcd,Cleawing/btcd,btcsuite/btcd,Dirbaio/btcd,danoctavian/btcd,FactomProject/btcd,decred/dcrd,jimmysong/btcd,CryptocurrencyCabal/htcd,Dirbaio/btcd,sysbot/btcd,ppcsuite/ppcd,mathiasrw/btcd,BitGo/prova,Roasbeef/btcd,dajohi/btcd,ltcsuite/ltcd,wallclockbuilder/btcd,runeaune/btcd,cjepson/btcd,jrick/dcrd,DanielKrawisz/btcd,CryptocurrencyCabal/htcd,jrick/btcd,btcsuitereleases/btcd,ppcsuite/ppcd-old,dajohi/btcd,davecgh/btcd,adiabat/btcd,toddfries/btcd,jcmincke/dcrd,aakselrod/btcd,qchain/btcd,adiabat/btcd,jimmysong/btcd,ppcsuite/ppcd,danoctavian/btcd,Dirbaio/btcd,cjepson/btcd,DanielKrawisz/btcd,Evil-Coin/Evil,dan-da/btcd,wallclockbuilder/btcd,jrick/btcd,cqswxj2013/btcd,davecgh/btcd,Evil-Coin/Evil,sysbot/btcd,FactomProject/btcd,qchain/btcd,tb00/btcd,Stash-Crypto/btcd,jongillham/btcd,runeaune/btcd,ppcsuite/ppcd-old,cqswxj2013/btcd,soapboxsys/ombfullnode,glv2/ppcd,mathiasrw/btcd,jongillham/btcd,aakselrod/btcd,Cleawing/btcd,jcmincke/dcrd,pkothbauer/btcd,glv2/ppcd,1d4Nf6/btcd,jrick/btcd,Roasbeef/btcd,hectorj/btcd,BitGo/prova,hectorj/btcd,tb00/btcd,btcsuite/btcd,stevenroose/btcd,Stash-Crypto/btcd,pkothbauer/btcd