added possibility to choose between holding and input registers

I added 'Function' parameter to modbus plugin config. It's value can be 'holding' or 'input'. Plugin then uses modbus_read_registers or modbus_read_input_registers function from libmodbus. Default value is holding. This does not work with legacy libmodbus.

Something like this should be added to modbus section of collectd.conf documentation:
Function holding|input
Specifies register type to be collected from device. Works only with libmodbus 2.9.2 or higher. Defaults to holding.