BUG/MINOR: lua: Duplicate map name to load it when a new Map object is created

BUG/MINOR: lua: Duplicate map name to load it when a new Map object is created

When a new map is created, the sample_load_map() function is called. To do so,
an argument array is created with the name as first argument. Because it is a
lua string, owned by the lua, it must be duplicated. The sample_load_map()
function will convert this argument to a map. In theory, after the conversion,
it must release the original string. It is not performed for now and it is a bug
that will be fixed in the next commit.

This patch may be backported to all supported versions, most probably as far as
2.1 only. But it must be backported with the next commit "BUG/MINOR: arg: Fix
leaks during arguments validation for fetches/converters".