allow committing transaction without a lock file

This is useful to allow front-ends to manage the lock file themselves, allowing them to ensure database consistency across multiple commands, e.g.:

 paclock --lock
 pacinstall --no-lock foo
 # foo is guaranteed to be installed until the db is unlocked
 paclock --unlock

changed the description

Example isn't great, but it illustrates the intent. I think expecting front-ends to be responsible about committing with FLAG_NOLOCK is reasonable, but if we're worried about accidents, we could add a separate flag.

I am interested in this, but also rather cautious! I'm going to propose something a bit more complex... Considerations I am thinking on:

• If a lock file is found, it is because another process has locked the database and still working, or is it a left over file
• Should FLAG_NOLOCK require a lock file is already in place?

We currently write the PID to the lock file, but I think we could write something useful here. E.g. a message of who created the lock file and when it was created?