Add 21 extra blank lines to the end of the 31-line DOOR.SYS file

Some doors/door kits actually require all 52 lines, but since they
(may) have problems with the values we populate on those additional
21 lines, just write them as blank when a program is configured for
the GAP (original) 31 line DOOR.SYS drop file. I doubt (and hope)
that no programs will care if their are more lines than they expect.