Modbus: allows modbus thread stack size customization with MODBUS_STACK_SIZE macro.

For example, add "-DMODBUS_STACK_SIZE=0x2000" to target CFLAGS to limit stack size to 128kB

<?xml version='1.0' encoding='utf-8'?>
<BaseParams xmlns:xsd="http://www.w3.org/2001/XMLSchema" IEC_Channel="0" Name="opcua_0"/>