Const-qualified parameters in exported functions

[pravorskyi]

Hi! I wonder if there is any chance to make const-qualification of parameters in some exported functions?

For example, change cfg_getopt(cfg_t *cfg, const char *name); to cfg_getopt(const cfg_t *cfg, const char *name);.

This will allow to write more const (pure) functions, especially in C++ projects. However, it will be require to change the error reporting function prototype from:
typedef void (*cfg_errfunc_t)(cfg_t *cfg, const char *fmt, va_list ap);
to
typedef void (*cfg_errfunc_t)(const cfg_t *cfg, const char *fmt, va_list ap);

While I see no reason to change cfg_t data inside the callback for custom error reporting, I suppose there might be use cases for this. The above changes may also break backward compatibility. But the tests of this project pass successfully after such changes on my local machine.