Add screenlock table

[directionless]

Adds a screenlock table. This is implemented by invoking osquery under launchctl runas to get the target user context.

This is implemented as requiring user, and not as iterating against the logged_in_users, because it feels more correct to keep that kind of logic in sql. Testing does not show incorrect data being returned. Though, if we uncover issues, this should be revisited.

It would be a cleaner to require the user specification as uid, and not user. But requiring joins against the user table becomes awkward. This may change in the furture.

Replaces: #618
Fixes: #614

[blaedj]

Should I expect this to work from make sudo-osqueryi-tables? I only get empty results, no matter which user I user as a constraint

[directionless]

sudo won't really work, no. Try make launchas-osqueryi-tables Though, we should chat on slack, you should be able to get your user's info.

[blaedj]

LGTM, as discussed elsewhere there are a lot of permission/path issues that are best tested in the beta channel