Support for converting OpenOptions to c_int on UNIX

[withoutboats]

AFAICT, there's no public API for converting OpenOptions to c_int on UNIX platforms, meaning you cannot use an OpenOptions to open a file yourself using a lower level API from C. For example, this means that I cannot just re-use the OpenOptions type from std in ringbahn, which opens a file using io-uring instead of using the open syscall.

Some way under std::os::unix to convert OpenOptions to c_int would be helpful to avoid duplication. Ideally it would be a method on OpenOptionsExt, but I'm not sure how that could be non-breaking since there is no obvious default implementation. It may just have to be a free function.

[Dylan-DPC-zz]

Reopening this because it is being reverted in #77090