Allow `ATTACKMOVE`s `ATTACK` to exclude specified targets

There is currently no reasonable way to block the `FIGHT` command's auto generated `ATTACK` commands or block the targeting of the unit itself that I'm aware of from the Lua API.

@saurtron
Would it be possible/difficult to make units with active `Fight` commands ignore a table of `UnitID`s?

The issue names the Lua API and the Fight/ATTACKMOVE command behavior; start by tracing how Fight generates ATTACK commands and how target UnitIDs are selected. Done means active Fight commands can ignore a caller-specified table of UnitIDs, including the unit itself, with behavior verified in the relevant command tests.

Written by the indexing model from the issue text.