Format Palisade solve-type moves in sensible ASCII.

Format Palisade solve-type moves in sensible ASCII.

The solve move stored in 'aux' by new_game_desc consists of printable
characters in the range '@' to 'O', each representing a 4-bit bitmap
of edges around a cell. But the one generated on the fly by
solve_game() was missing out the 0x40 bit and just returning
characters in the range ^@ to ^O - which would not only have been
horrible if you found such a string in a save file, but also meant
that a game with any completely borderless square would have a
solution move string terminating early due to the ^@, causing
execute_move() to reject it.

Example: ./palisade --test-solve --generate 1 5x5n5#12345-37 now
succeeds, where previously it failed an assertion.