Replace OFlag in FileEvent::Create with custom enum

[krs85]

Currently, we have:

FileEvent::Create(OFlag, SyscallOutcome)

FileEvent::Open used to be the same way, but we replaced the opaque OFlag with a nice custom enum. We should do this for FileEvent::Create as well:

FileEvent::Create(CreateMode, SyscallOutcome) where CreateMode is:
enum CreateMode { Create, Excl, }

• Change FileEvent::Create.
• Update generate_open_syscall_file_event() for O_CREAT and O_EXCL cases.
• Update generate_file_preconditions().
• Update generate_file_postconditions().