userns: clear and save id_map (v2)

Otherwise (a) there is a memory leak when using user namespaces and
clearing a config, and (b) saving a container configuration file doesn't
maintain the userns mapping.  For instance, if container c1 has
lxc.id_map configuration entries, then

python3
import lxc
c=lxc.Container("c1")
c.save_config("/tmp/config1")

should show 'lxc.id_map =' entries in /tmp/config1.

Changelog for v2:
   1. fix incorrect saving of group types (s/'c'/'g')
   2. fix typo -> idmap->type should be idmap->idtype

Reported-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Dwight Engen <dwight.engen@oracle.com>
Tested-by: Dwight Engen <dwight.engen@oracle.com>