config: make it possible to run several instances of configuration check at once

config: make it possible to run several instances of configuration check at once

Before, the status of the configuration check (config_def_check fn call)
was saved directly in global configuration definitinion array (as part
of the cfg_def_item_t/flags)

This patch introduces the "struct cft_check_handle" that defines
configuration check parameters as well as separate place to store
the status (status here means CFG_USED and CFG_VALID flags, formerly
saved in cfg_def_item_t/flags). This struct can hold config check
parameters as well as the status for each config tree separately,
thus making it possible to run several instances of config_def_check
without interference.