Add pidfd_getfd syscall to `os` module

Since Linux 5.6 we have pidfd_getfd which is quite a close cousin to already supported in Python pidfd_open.

Let me know if you think it’s a good idea :slight_smile:

BR,
Patryk

FTR: Use case: pidfd_getfd allows for big simplification of sharing FDs (e.g. File Descriptor Transfer over Unix Domain Sockets | by Cindy Sridharan | Medium and corresponding Seamless file descriptor transfer between processes with pidfd and pidfd_getfd | by Cindy Sridharan | Medium)