Refactor to avoid client code duplication

[rolandweber]

Some of the code for the Sockets client and Java RMI client is identical, or nearly so:

• MsgBoardCommandDispatcher class and MsgBoardClientHandler interface
• *BackendHandler interface and *Impl class, the handling of hostname and port
• Catalog class and CatalogData properties, the command-line argument messages
• some of the console output messages, too

Package name? pityoulish.cmdline.mbclient would match the scope. But #11 introduces a common server-side package which is not specific to command-line arguments. So maybe pityoulish.mbclient here and pityoulish.mbserver there?

• [x] Consolidate the duplicated code into a common client utility package.
• [x] Update the Code Overview sections of the exercise descriptions.

[rolandweber]

Follow-the-Board clients (#51, #5) also require *BackendHandler and *Impl. They're now re-using the code from the client packages though.