This patch introduces new element <idmap> for
user namespace. for example
<idmap>
    <uid start='0' target='1000' count='10'/>
    <gid start='0' target='1000' count='10'/>
</idmap>

this new element is used for setting proc files
/proc/<pid>/{uid_map,gid_map}.

This patch also supports multiple uid/gid elements
setting in XML configuration.

We don't support the semi configuation, user has to
configure uid and gid both.
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>