Document buffer overrun for long land paths

Document buffer overrun for long land paths

BestLandPath(), BestDistPath() and best_path() are unsafe by design:
they take a path[] argument without a size, and blindly assume there's
enough space.  When that's wrong, bp_path() overruns the caller's
buffer.

move_ground() and assemble_dist_paths() provide space for 512
characters.  best(), dist(), path(), att_reacting_units(), s_commod()
and do_unit_move() provide space for 1024 characters.

A malicious player can arrange paths longer than that, but it takes a
lot of land.

BestAirPath() and BestShipPath() also take a path[] argument without a
size, but they're actually safe: bestownedpath() writes at most 100
(MAXROUTE) characters, perform_mission_bomb() provides space for 512,
sorde(), getpath(), do_unit_move() and nav_ship() for 1024.