Add support for external control via a Unix domain socket.

This feature allows an external program to inject output into and
listen to input from a serial port via a Unix domain socket (path
specified via the -S/--socket command line flag, or the socket
config file option) while tio is running. This is useful for ad-hoc
scripting of serial port interactions while still permitting manual
control. Since many serial devices (at least on Linux) get confused
when opened by multiple processes, and most commands do not know
how to correctly open a serial device, this allows a more convenient
usage model than directly writing to the device node from an external
program.

Any input from clients connected to the socket is sent on the serial
port as if entered at the terminal where tio is running (except that
ctrl-t sequences are not recognized), and any input from the serial
port is multiplexed to the terminal and all connected clients.

Sockets remain open while the serial port is disconnected, and writes
will block.

Example usage 1 (issue a command):

  echo command | nc -UN /path/to/socket > /dev/null

Example usage 2 (use the expect command to script an interaction):

  #!/usr/bin/expect -f

  set timeout -1
  log_user 0

  spawn nc -UN /path/to/socket
  set uart $spawn_id

  send -i $uart "command1\n"
  expect -i $uart "prompt> "
  send -i $uart "command2\n"
  expect -i $uart "prompt> "

src/main.c

@@ -30,6 +30,7 @@
 #include "error.h"
 #include "print.h"
 #include "signals.h"
+#include "socket.h"
 
 int main(int argc, char *argv[])
 {
@@ -71,6 +72,9 @@ int main(int argc, char *argv[])
     if (option.log)
         log_open(option.log_filename);
 
+    /* Open socket */
+    socket_configure();
+
     /* Enable ANSI text formatting (colors etc.) */
     print_enable_ansi_formatting();