Add support for the Voltcraft M-4650CR.
Add support for the Metex M-4650CR.

(the Voltcraft M-4650CR is a rebadged Metex M-4650CR)
metex14: Handle a few missing cases for overflow.
hameg-serial: Initial driver skeleton.
serial: Add function to parse serial options.
hameg-serial: Add initial scan for HMO devices
usb: Add autofind for serial usb devices
hameg-serial: Refactor out the probing logic.
hameg-serial: Add USB auto discovery
serial: Fix return value of serial_read() fow Windows.
scpi: Add helper functions for scpi communication
hameg: Use common SCPI functions for discovery.
hameg: Add get_device_type helper function.
scpi: Add helper functions: string to number.
scpi: Add function to strictly parse bool strings.
scpi: Add more functions (getting int/bool/float).
scpi: Add function to get an array of floats.
Move sr_ato* functions to strutil.c
Document sr_ato* functions.
Document sr_parse_strict_bool.