server/: Institute `address family table'; contemplate multiple sockets.

This is a slightly oddly-shaped change which lays important groundwork
for the future.

  * Firstly, it creates a table of address families, currently not very
    interestingly since there's only one, but this will be an essential
    tool for adding IPv6 support later.

  * Secondly, it turns the peer module's `sock' into a global vector
    `udpsock' of UDP sockets, possibly one for each of the supported
    address families.  There's no real change here, because there's only
    one address family known, but the `port' command has grown an
    address-family argument in case they have different ports.  To make
    this work, each peer now keeps track of the index of the socket it
    should use for transmitting messages.