Adding TCP server handling to libmodbus

[kjeldflarup]

This is the same as for 3.1.4 #341

When creating a modbus server you must create your own
TCP / socket server for libmodbus.
This commit aims to integrate this handling into libmodbus.

Features:

• Function to create a TCP server context where you specify port,
  ip and max_connection.
• Handle function which must be called from a task or loop.
• Destroy function with cleans up and correctly closes all current
  connections.
• Function to change select timeout

Uses the modbus specifications method of 'disconnecting the
last used connection if max_connection is reached' when
accepting new incoming connections.

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