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