fixup 7e39cbc3a5ff8ae0ab585b106564f359f249b042

whiteboard->allow_end_turn become unnegated... resulting that the
user cannot end the turn
add an operator bool() to unit_map::iterator

Semantically equivalent to `unit_map::iterator::valid()`
Uses safe bool idiom when c++11 is not enabled
move "set_menu_item" to be implemented via lua rather than C++