samples: show race-free pidfd metadata access

This is a sample program showing userspace how to get race-free access
to process metadata from a pidfd.  It is rather easy to do and userspace
can actually simply reuse code that currently parses a process's status
file in procfs.
The program can easily be extended into a generic helper suitable for
inclusion in a libc to make it even easier for userspace to gain metadata
access.

Since this came up in a discussion because this API is going to be used
in various service managers: A lot of programs will have a whitelist
seccomp filter that returns <some-errno> for all new syscalls.  This
means that programs might get confused if CLONE_PIDFD works but the
later pidfd_send_signal() syscall doesn't.  Hence, here's a ahead of
time check that pidfd_send_signal() is supported:

bool pidfd_send_signal_supported()
{
        int procfd = open("/proc/self", O_DIRECTORY | O_RDONLY | O_CLOEXEC);
        if (procfd < 0)
                return false;

        /*
         * ...