New options for lead indicators

[Kiloku]

Adds a new game_settings.tbl option: $Lead indicator behavior, which can be set to default, multiple or average. Also adds a new flag to weapons.tbl: don't merge lead indicators

When $Lead indicator behavior is set to default (or unset), lead indicators act just like in retail, meaning when you link them, only the indicator for the one with the farthest range will be drawn. When set to multiple, a lead indicator is drawn for all primary weapons in range when you have them linked.
When set to average, a single lead indicator is drawn in the average of the position where each of them would usually be drawn.
Invalid selections will revert to default and give a warning.

When Weapon flag don't merge lead indicators is set for a homing secondary, its lead indicator will be drawn as if it was a dumbfire missile.