Provide LSM hook for access()

Currently one cannot use access() to see if the call would be blocked by the LSM. It would be nice if this was in place so application developers could use a standard method to determine access instead of resorting to looking up sandbox variables or trying to open files in multiple ways and falling back. For example, online accounts may want to open the accounts.db database as read/write if the process is unconfined and read-only otherwise. Currently it is trying to open read/write and falling back to read-only; having access() available to say what the LSM would do would be helpful.