WiredTiger's cursor validity checking function has grown a lot since it was first introduced. It now is handling many different possible use cases and it's implementation has become bloated and messy. Some of the issues with it are:

• Occasionally a key is passed.
• Occasionally a recno is passed.
• The CURSTD_KEY_ONLY flag is checked outside the function.
• The logic inside cursor valid depends on early returns from the function to make sure it doesn't do the wrong thing.
• It handles FLCS, VLCS and rowstore all at once.

We should refactor the function to reduce its complexity and provide a significant cleanup to the btcur logic.