Save/restore the PRGS at checkpoints (necessary for internal demons).

For some reason, I believed that the source dir containing the main
program need to be named bin for dune to work smoothly. But actually,
it seems to work fine with src, so let's go for it.

This is necessary to be able to use sasa with luciole.  Indeed, in
rdbg/lurette, luciole always plays first. Hence if we want sasa to
play after luciole, we can use the option.