config: Explicitly handle help requests.

[jholdstock]

Checking for --help as an explicit step before parsing any other configs makes the code more intuitive by removing a convoluted bit of error handling.

It also enables the IgnoreUnknown option to be used whilst parsing for help, which ensures the presence of --help will always result in the help message being printed. This fixes a minor inconsistency where the help message would be printed if the flag was placed before an invalid config, but placing it after would cause an invalid config error to be written instead. For example, dcrd --help --fakeflag vs dcrd --fakeflag --help.

[davecgh]

Looks reasonable. I'll do some testing before approving.

[davecgh]

Tested it works as expected:

$ ./dcrd --fake -h
Usage:
...
exit status 0
$ ./-h dcrd --fake
Usage:
...
exit status 0
$ ./dcrd --fake
unknown flag `fake'
Use dcrd -h to show usage