Always show help on help flag.
Always show help on help flag.

This commit resolves a minor issue where an error in the config file would
prevent the help from being shown until the config file error was
resolved.

This results in the following behavior:

- With a malformed header:
  $ ./btcd
  Error parsing config file: ~/btcd/btcd.conf:14: malformed section header
  Use btcd -h to show usage
- With an invalid option:
  $ ./btcd
  Error parsing config file: unknown option: bogus
  Use btcd -h to show usage
- Invoking help with an error in the config file:
  $ ./btcd -h
  Usage:
  ...
- Invoking with an invalid command line option:
  $ ./btcd --bogus=bogus
  unknown flag `bogus'
  Use btcd -h to show usage

ok @jrick

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