:mod:`os` and :mod:`signal`: Use glibc functions instead of ``syscall()``:
``pidfd_open()``, ``pidfd_getfd()`` and ``pidfd_send_signal()`` (glibc 2.36),
``gettid()`` and ``getdents64()`` (glibc 2.30), and ``getrandom()`` (glibc
2.25). Patch by Victor Stinner.
