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