The board is 63x40 or 2520 squares.
Each horizontal row of the board represents 9 bytes of efdtt.c. Each
byte is encoded as 7-bit ASCII. Covered squares (where the bombs are)
represent ones, and uncovered squares represent zeros. This is enough
to encode 2520/7 = 360 bytes of the source.
The remainder of efdtt.c is encoded on top of the covered squares,
using question marks to represent ones. The file is null terminated
(character zero).