POSIX XXXat() methods

Hi

Eshan has been looking for generic implementations of the 15 POSIX methods
like openat(), mkdirat(), etc. He has searched the normal places I know of
and hasn't found implementations of most of them that don't just call a
dedicated system call. Here is his analysis.

https://docs.google.com/spreadsheets/d/1nEniQOs9TxDy5fb7P0_XFg7VknQDldG1BDFWSoqdKg0/edit?usp=sharing

It appears that many could be implemented generically but with possible
overhead of saving current working directory, changing, returning, etc. and
likely race conditions.

Does anyone have any ideas on where to get generic implementations to port?
Or implementation strategies to write OS agnostic versions?

Thanks.

--joel