Introduce limitations on combinations of box.cfg parameters.
* Add restriction on log type file and log_nonblock=true.
* Add restriction on log type syslog and log_format json.
* Each restriction creates error in case of its violation.
* Change log_nonblock default value to nil, which means default values
of log_nonblock corresponding to type of logger.
* Add box_getb function for receiving bool parameters from cfg.

Relates #3014 #3072