New feature: `-k` flag (keep streaming new data to TCP connections)

[mik3y]

This change set adds the -k flag, which affects the TCP listener. When -k is given, the TCP socket will continue to publish newly-decoded messages to any connected clients.

I've tried to keep unnecessary diffs to a minimum. Here's a summary of the major parts of the change:

typedef struct t_sockIo {...}

The struct which tracks open TCP clients is extended to add int msgpipe[2]. These will store two file descriptors, as created by pipe(2), for the decoder thread to publish to the connection thread. This pipe is created when a new client connects, and destroyed upon disconnect.

add_nmea_ais_message(..)

When a new message is decoded, and the global _tcp_stream_forever option is set, this method is extended to publish the message to all connected clients. It does do by write(2)ing to the msgpipe fd.

handle_remote_close(..)

This method continues to service remote TCP clients. After publishing buffered messages (existing functionality which is unchanged), it waits on a blocking pselect(2) loop for one of two things to happen:

• New data is available on the read fd of msgpipe. It writes it out to the tcp socket.
• New data arrives from the tcp socket. By previous convention, this causes the server to close the socket.
  • It's not clear to me if that behavior remains necessary here; i.e., we could ignore reads and let the client close when it wants.

When -k is not specified, no data is ever published to msgpipe; so the existing behavior remains intact.

Other/unrelated changes

There are two other commits here which address minor/unrelated things I ran into. It might be easier to review by looking at commits one at a time, which should be coherent.

Closes #33.