Improve usage display a little.
Improve usage display a little.

This commit makes use of the new default-mask go-flags option in
conjunction with delaying the usage display until after the config file is
parsed.  This has a couple of nice properties such as showing the actual
values that will be used as loaded from the specific config file instead
of the defaults specified in btcd itself, and also allows any config file
parsing errors to be shown prior to displaying the usage.

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