[libgpiod]: feature request: API functions, const correctness

["Hollerer Franz, Schrack Seconet AG, Entwicklung" <f.hollerer@xxxxxxxxxxxxxxxxxxx>]

Dear libgpiod-team,

I have a request, and it would be great if you could consider it for future versions of libgpiod. Within the current API it is not obvious which function parameters are input-only parameters, and which parameter are output parameters, i.e., will be modified.

I think it would be helpful if the API is const correct. As example, I think

> struct gpiod_line_info *gpiod_chip_get_line_info(struct gpiod_chip *chip,
> 						 unsigned int offset);

should be changed to

> struct gpiod_line_info *gpiod_chip_get_line_info(const struct gpiod_chip *chip,
> 						 unsigned int offset);

making it clear that the chip object is an input parameter which is not changed by the API function. Same argument applies for many other API functions...

Thanks for consideration & Best regards,

Franz Hollerer