Implement logged_in_users.

Fixes #9.
Merge pull request #493 from wxsBSD/issue_9

Implement logged_in_users.
Towards predicate constraint checking
Organizing affinity types into tables.
Updating table generators to use QueryContext
Rebuild generated files when templates change.
Predicate constraints for Linux
Predicate constraints for FreeBSD
Improve kextstat/startup_items code and perf
Moving tables definitions into core/tables.cpp
Rename/improve bash_history to shell_history
Improve and use constraints for various OSX tables
Improve OSX apps table
clang-format on feature-predicate updates
Add query constraints to logged_in_users