Add further controls to lazy restoration

At the moment drreg users do not have control over lazy restoration. In particular, lazy restoration is always done at the end of a basic block.

Advanced drreg users might want finer control, e.g., calling an API function like drreg_restore_all() that restores at a particular point all spilled registers maintained by drreg. Introducing a property that turns off drreg end-of-block restoration should also be included.