zdtm: Add ability just to start the test

When running test with ns/ prefix zdth.sh does complex preparations.
Make it possible to make them and let started process ready for
manual investigation.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
util: Prepare proc opening helpers to open any files

We have a set of routines that open /proc/$pid files via proc service
descriptor. Teach them to accept non-pids as pids to open /proc/self/*
and /proc/* files via the same engine.

Signed-f-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
restore: Open /proc/self/maps via helpers

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
restore: Open /proc/sys/kernel/ns_last_pid via helpers

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
restore: Do fchroot() via proc helpers

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>
zdtm: Add test for mount namespace w/o mountpoints

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>