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/socket.h

@@ -0,0 +1,31 @@
+/*
+ * tio - a simple serial terminal I/O tool
+ *
+ * Copyright (c) 2014-2022  Martin Lund
+ * Copyright (c) 2022  Google LLC
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+#pragma once
+
+#include <stdbool.h>
+#include <sys/select.h>
+
+void socket_configure(void);
+void socket_write(char input_char);
+int socket_add_fds(fd_set *fds, bool connected);
+bool socket_handle_input(fd_set *fds, char *output_char);