Function setting_set 

pub unsafe extern "C" fn setting_set(
    context: *mut c_context,
    key: *const c_char,
    value: *const c_char,
) -> err

@brief Sets a setting in the nix global configuration.

Use “extra-” to append to the setting’s value.

Settings only apply for new State%s. Call nix_plugins_init() when you are done with the settings to load any plugins.

@param[out] context optional, Stores error information @param[in] key The key of the setting to set. @param[in] value The value to set for the setting. @return NIX_ERR_KEY if the setting is unknown, or NIX_OK if the setting was set successfully.