linux: Add pidfd_open

This was added on Linux 5.3 (32fcb426ec001cb6d5a4a195091a8486ea77e2df)
as a way to retrieve a pid file descriptors for process that has not
been created CLONE_PIDFD (by usual fork/clone).

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

sysdeps/unix/sysv/linux/sys/pidfd.h

@@ -0,0 +1,33 @@
+/* Wrapper for file descriptors that refers to a process functions.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _PIDFD_H
+
+#include <fcntl.h>
+#include <bits/types/siginfo_t.h>
+
+#define PIDFD_NONBLOCK O_NONBLOCK
+
+/* Returns a file descriptor that refers to the process PID.  The
+   close-on-exec is set on the file descriptor.
+
+   The FLAGS argument is reserved for future use, it must be specified
+   as 0.  */
+extern int pidfd_open (__pid_t __pid, unsigned int __flags) __THROW;
+
+#endif /* _PIDFD_H  */