The head of the propagation queue was not begin properly reset.

Adding some debugging functions.

Fixing small bugs.

- Implementation of bookmark, unbookmark and rollback procedures.
- Minor changes.

- New implementation of watcher lists.

- Adding a compile time option to use floats for var activity (now it can be either ‘double’, ‘float’ or ‘unsigned’ (default))
- Adding vector of ‘float’
- Adding an option to configure the ratio of learnt clauses to be kept in clause database at each reduction (0 means no reduction).
- Other small changes.

The command is ‘satoko’