Revamp ggo help.

Revamp ggo help.

This adds usage and description fields to struct ggo_help and
changes ggo_print_help() to optionally print these.

The boolean detailed_help flag of ggo_print_help() is replaced by a
bitmask which lets the caller specify what to print. Four pre-defined
masks are used to print the normal help, the detailed help, the help
for modules (receivers, filters, writers) and the detailed module help.

The new macro DEFINE_GGO_HELP can be employed to create a struct
ggo_help from a gengetopt structure.