Perform display handler check before issues RPC.
Perform display handler check before issues RPC.

This commit modifies the command handler in btcctl to check the existence
of a display handler before issues an RPC command.  This prevents a round
trip to the server if there is no display handler.

Also, fix a couple of comments while here.

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