Security, birth, getters for unit, hivemind.

You can give birth, and a security flaw was fixed in tile::walkable.
Also, secure public getters were created (X macros) to access unit's
members, and the unit's hivemind's members, the hivemind's members, and
the hivemind's unit's members.
Also, tile's getters were modified, because they didn't actually get
anything.