Added admin/server commands

Peers with admin privileges (todo!) can run these commands to control
the server.

Commands

Disconnect a peer by index
adm dc -i [0 - MAX_PEERS]

Disconnect a peer by PID
adm dc -p [PID]

Added some extra functionality to host: idx_by_pid(), and other
miscellaneous things like a message printed to stdout when a new peer
connects, with slot/PID info.