WIP: Support Access Control Flags

[sosheskaz]

This is a rough draft to enable access control flags.

I am not very familiar with the Keychain API, nor Swift/Go integration, so this implementation is likely flawed.

1. I am not sure how to easily or safely test this change, especially given that by its nature it cannot be run non-interactively.
2. I could not figure out how to reference the flag values (like SecAccessControlCreateFlags.userPresence so I re-created the values instead.
3. I am not sure if using the native type (SecAccessControlCreateFlags which seems to boil down to a ulong) in Go. Is a conversion layer better than direct referencing like I am doing?

Most of what I am doing here is guessing... wonder if any insight can be given on further changes that should be made, or if a more experienced contributor could help me get this over the finish line.

[allen-branch]

I spent some time trying an approach similar to yours and I believe I was very close, but because the binary does not have any entitlements, it will fail with this error, https://developer.apple.com/documentation/security/errsecmissingentitlement.

I can't confirm that's the reason it's failing though due to now having an apple developer account to try it in a signed app with proper entitlements.

See master...allen-branch:go-keychain:master