Implement feature request from access_d: ZONE_BLIND threshold

If the ZONE_BLIND option is specified in the sbbsecho.cfg file and it is
followed by a decimal number (e.g. "ZONE_BLIND 4"), then all zones less
than or equal to that number (the threshold) will be treated as a single zone
and all zones greater than that number (the threshold) will be treated as
separate zones (normal FTN behavior). The default zone blind threshold is
0xffff (all possible zones), when zone blind is used.