Server: Added ShellUser and ShellUsers

ShellUser and the ShellUsers collection manage the state of all the
shell connections open to a server.
Client|Server: Segregated functionality in sys_network, added ShellUsers
Documentation|libdeng2: API documentation additions
libdeng2: LegacyNetwork can relinquish control of a socket
Shell|Server: Minimal shell connection is up and running

A shell can now open a link, send console commands, have them executed
on the server, and receive log entries from the server.