zdtm: Make it possible for test to get ZDTM_NEWNS variable

I will need to make cgroup test behave slightly differently
when it's in and out of ns/ run. To do so it's handy to use
the ZDTM_NEWNS variable set by zdtm.sh

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
mnt: Strip commas from options string

Not all filesystems like it. Other than this options in the
image just look cleaner.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
files: Fix open_path() to provide mntns root fd to callbacks

This fixes the support for fifo-s in mount namespaces and
makes it easier to control the correct open_path() usage in
the future.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>