Spring.GetUnitWeaponHaveFreeLineOfFire has terrible interface

[sprunk]

• it has four different argument sets, making it hard to intuit what is going on.
• worse, arguments overlap between the sets, but are on different positions. For example in one set, 3rd arg is a target unitID. In another, you can also pass target unitID but it's now the 6th arg, with 3rd arg being the source X coordinate.
• none of these are documented.
• it seems to have a similar problem as Spring.ShareResources(teamID, "units", nil) breaks #1826 where an explicit nil changes behaviour (by making it interpret args according to a different set).
• there doesn't seem to be a set that would check against the weapon's current target. There is one that seems to check line of fire towards {0, 0, 0} coordinates, oversight?