Perfect foresight: LBJ now available under stack_solve_algo=1 (with/without block/bytecode)

Previously, LBJ was available:

– under stack_solve_algo=6 when neither block nor bytecode were present
– under stack_solve_algo=1 with either block or bytecode (but the documentation
  was not making it clear that it was LBJ)

This commit merges the two values for the option, and makes them
interchangeable. LBJ should now be invoked with stack_solve_algo=1 (but
stack_solve_algo=6 is kept for compatibility, and is a synonymous).