* Allow tio to connect to socket

   After some more consideration I think it makes sense to support connecting to a
   socket as that will make tio be able to both serve a serial port via a socket
   and connect to it - it will be an end to end solution. In short we will be able
   to do the following:

   Host serial port on socket (existing feature):
    $ tio --socket unix:/tmp/tio-socket-0 /dev/ttyUSB0

   Connect to same socket (new feature):
    $ tio unix:/tmp/tio-socket-0

   Besides a bit of refactoring the following required changes spring to mind:

    * Socket mode and type of socket should be activated via device name prefix. For example:
      * UNIX socket: tio unix:<filename>
      * TCPv4 socket: tio inet:<ip>:<port>
      * TCPv6 socket: tio inet6:<ip>:<port>
    * If no port number defined default to 3333
    * Mapping flags INLCR, IGNCR, ICRNL needs implementation for socket mode
    * Error messages should just say "device" instead of "tty device" etc.
    * Remove other tty'isms (tty_write() should be device_write() etc.)
    * In session key commands that do not work in socket mode should either not be listed or print an error messages if used.
    * All non-tty features should continue work (auto-connect etc.)
    * Shell completion script update
    * Man page update


 * Split I/O feature

   Allow to split input and output so that it is possible to manage these
   independently.

   The general idea is to redirect the output stream on the socket port number
   specified but then redirect the input stream on the same port number + 1.

   Example:

    $ tio /dev/ttyUSB0 --socket inet:4444,split-io

   Will result in output stream being hosted on port 4444 and input stream
   hosted on port 4445.

   For file sockets something similar can be arranged:

    $ tio /dev/ttyUSB0 --socket unix:/tmp/tio-socket-0,split-io

   Will result in output stream being hosted via /tmp/tio-socket-0 and input
   stream hosted via /tmp/tio-socket-0_input

 * Websocket support

   Extend the socket feature to redirect serial I/O to websocket on e.g. port
   1234 like so:

    $ tio --socket ws:1234

   Use libwesockets to implement feature.