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, which happened to be unnecessarily duplicated in three places.
Moving it to a function in the internal package makes it reusable by
multiple binaries.
This 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, `vspd --help --fakeflag` vs `vspd
--fakeflag --help`.
