replace the specific clone options by a -s option
with the Ored list of namespaces to be cloned.

take the opportunity of this change to have
the -u <uid> option to not automatically set
the clone flag to CLONE_NEWUSER but to be dependant
of the new -s USER option.
Signed-off-by: Michel Normand <normand@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>