Unit Navigation: Updated following code. The followed unit is no longer blocked by the following units, so it can move away when surrounded. The followers will relax their position and move out of the way when other units come close. Introduced the navigator flag GF_ATGOAL_RELAX for this purpose.

This is not applied to the attack following code, in this case units will always block other units when in range of the enemy target unit.

Also removed the unused variable m_vLastPosition.