Friendlier usage output.

1. The commands dictionary is extended with a command group, as well as
   the one-liner synopsis and help message. When the list of these
   one-liners get output, they are now output in sorted order within
   each group.

2. Instead of having the syntax on the leftmost 40 columns and the
   description on the rightmost 40 columns, causing many commands to
   span more than one line total, we now display the syntax on a whole
   line, and the description on the next one, indented by a tab.

   This should hopefully improve readability (especially for complex
   syntax lines).

3. The command synopsis is now passed to the command handler. This
   allows error messages in case of invocation error to be consistent
   with the output of `pdnsutil help`.