Introduce lsu.{c,h}, implement help --long for para_server.

This adds the --long option to the server help subcommand. The former
help output becomes the long help while the short help text is shown
if --long is not given.

Although only the help command of para_server is converted in this
patch, the new functionality is implemented in a generic way so
that the help commands of para_audiod and para_play can use the same
implementation. Those will be converted in subsequent patches.

t0004 parses the help output and thus needs to be changed to include
--long.