libdeng2|Network: Added Beacon, UDP-based peer discovery mechanism

The Beacon class acts as a UDP broadcast receiver that will be used
for replying to clients' discovery queries.
libdeng2: Improved Record, Block, and ByteRefArray

More versatility and missing assignment operators.
Server: Use a Beacon to announce presence of a server locally

The beacon is supplied with an up-to-date serverinfo_t so that clients
can immediately know what is going on at the server.
libshell: Added ServerFinder

shell::ServerFinder uses a Beacon to discover local servers and parses
the Record contained in the servers' beacon advertisement message.
Shell: Testing a ServerFinder

The proper usage of a ServerFinder is to have it running in the
background continuously and insert the found servers in a menu.