Support 31 line and 52 line door.sys file creation

Synchronet has always created the 52 line door.sys file, however, the original format was only 31 lines (e.g. https://github.com/wwivbbs/wwiv/wiki/DOOR.SYS-format). Since some of the additional fields might cause some door programs/frameworks to choke, a configuration option (per door) to only generate a 31-line file could be useful.