Better help messages

[hydrargyrum]

The manpage tomb(1) mentions "tomb dig" but doesn't tell what are the arguments and in what order.

% tomb dig        
tomb  .  Commanded to dig tomb 
tomb [E] Missing path to tomb
% tomb dig my.tomb
tomb  .  Commanded to dig tomb my.tomb
tomb [E] Tombs can't be smaller than 10 mebibytes
% tomb dig my.tomb 10
tomb  .  Commanded to dig tomb my.tomb
tomb [E] Tombs can't be smaller than 10 mebibytes
% tomb dig -h     
tomb  .  Commanded to dig tomb 
tomb [E] Missing path to tomb

tomb dig -h should give the subcommand help and the manpage should mention expected args for a subcommand.

[jaromil]

planning to do a usage_help() internal function called by fatal errors printing out a usage example string for each subcommand when failing.

subcommand help is a bit too much work refactoring the old cli, but will take into account for v3