battle_calcs: provide default value for resistance modifiers

This is done for cases when the value cannot be read from the unit_info
table, which might happen in some UMC.  This fixes a rare bug affecting
the Fast Micro AI attack evaluation and probably some other AIs.
next_hop(): bug fix for ignoring units when ignore_units=true is set

A bug in the previous version lead to them not being ignored when they
were on the last hex of the next hop.
ai_helper.get_attacks: improve check for units in the way

Check whether the unit in the way has an unoccupied hex to move to.
Previously, it was only checked whether its reach was >1, which
sometimes can include only hexes occupied by units on its own side,
making the actual move impossible.