Improve `ipa --help` output
    
    Fix the usage string to match actual usage.
    
    Add command description.
    
    Put information about `ipa help topics` etc. to the epilog,
    instead of using empty option groups. Use a custom formatter
    to preserve newlines.
    
    Add the -h/--help option manually to ensure consistent case
    (capital S).
    
    Part of the effort for https://fedorahosted.org/freeipa/ticket/3060