O_EMPTY_PATH for openat

[swick]

If one has an O_PATH fd and wants to re-open an operable version, there are two options currently:

• If fd refers to a directory openat (fd, ".", O_DIRECTORY | ...);
• otherwise open ("/proc/self/fd/$FD", ...);

The dependency on procfs is not great. It would be nice if openat worked on an empty path.

FreeBSD has O_EMPTY_PATH for openat, and fstatat and others have AT_EMPTY_PATH.

[brauner]

Seems like a good idea to me in general. Can you add an pull request so it's added to the list?

[swick]

#48