namespaces: add switch_ns_by_fd

It's like switch_ns, but it gets a namespace file descriptor instead of pid.

travis-ci: success for net: simplify restore of macvlan-s (rev2)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>