There's no such thing as fchroot() in Linux, but we need to do
chroot() into existing file descriptor. Before this patch we did
this by chroot()-ing into /proc/self/fd/$fd. W/o proc mounted it's
no longer possible, so do this like

fchdir(proc_service_fd);
chroot("./self/fd/$root_fd");
fchdir($cwd_fd);

Thanks to Andrey Vagin for this trick ;)
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>