Replace PWD with CURDIR in Makefile

... so that it is possible to invoke make with -C flag from another
directory.
More dpor tests
Add exhaustive scheduling tests
Optimal DPOR
Bug fixes
Re-implement source-set DPOR
Refactor the scheduler code
Optimize recent_dependency_cv
Restore classic and simple source-set based DPOR algorithms
Cleanups in testsuites
One more test for sleep-set blocking detection
Fix spawn_opts when 'monitor' is an opt