nsenter: use pidfd to enter target namespaces

nsenter: use pidfd to enter target namespaces

The typical use case is to enter namespaces of the task (--target
<pid>). The original nsenter opens /proc/<pid>/ns/* files and uses the
file descriptors to enter the namespaces by setns(). The recent kernel
allows using the pid file descriptor instead of the files in /proc,
making it possible to enter multiple namespaces with one setns call.
This solution reduces the number of syscalls (open+setns for each
namespace), removes the dependence on /proc, and allows entering
nested namespaces.

This commit should be backwardly compatible, meaning it can be used on
systems without pidfd_open(). Explicitly specified namespaces by
filenames are still supported, and user namespaces are still entered
first/last according to permissions privileging/deprivileging.

Addresses: https://github.com/util-linux/util-linux/pull/301
Signed-off-by: Karel Zak <kzak@redhat.com>