Add support for 31-line DOOR.SYS file format (alternative to 52-line version)

Removed UTIDOOR.TXT file format: nobody uses this drop file format and we
removed the UTI driver long ago, so this was just an artifact.

If you previously had an external program (door) configured in SCFG to use
the "GAP DOOR.SYS" file format, SBBS will create the same (52 line) format
as before (no change).

But for doors that have compatibilty issues with the 52-line DOOR.SYS file
created by Synchronet (e.g. Thunder cat V3.30), a sysop can now choose "GAP
(original)" for the drop file type and Synchronet will create a 31-line
DOOR.SYS file instead.