previously one could use config options without arguments, or
config options with wrong names, and the program did not complain.
now you have to use correct names for config options and only
options with type ARG_NONE can be without arguments.
hardcoded default config file and config string are supposed
always correct so there's still no error check on their parsing.
however for any config options passed from command line with -f or
-C all the checks are enabled. if there are some syntax errors or
options without value the process exits with errors, printing
informations on all the found configuration errors.