Insert lua_unit's contained pointer directly

Not copying the unit improves the speed of wesnoth.put_unit by two orders
of magnitude from lua's perspective.

To do this, we have to take ownership of any owned pointer from lua_unit.
We do this by not running lua_unit's destructor, but just overwriting the memory it's in.
Give lua_unit a put_map member function

This allows us to remove the destructor-avoidance hack in wesnoth.put_unit
and simplify the function's handing of various lua_unit types.
Merge branch 'lua_put_unit' (PR #158)
Fix bug #21491: fix drag+drop for unit movements
Add players changelog entry for drag+drop fix.
Fix strict compilation
Merge pull request #161 from techtonik/master

scons: Format output for readability
scons: Explain out-of-tree build and Repository