`Spring.SetUnitShieldState` has poor multiple interface

It is one of:
* `Spring.SetUnitShieldState(unitID, number weaponNum, number power)` -> sets power for the given shield.
* `Spring.SetUnitShieldState(unitID, number weaponNum, bool enabled)` -> sets whether given shield is enabled.
* `Spring.SetUnitShieldState(unitID, number power)` -> sets power for the primary shield.
* `Spring.SetUnitShieldState(unitID, bool enabled)` -> sets whether primary shield is enabled.

This is a pretty unfriendly interface which is a nightmare to document.

Add functions that do one specific thing (keeping the existing one for back compat):
* `Spring.SetUnitShieldPower(unitID, number power, number? weaponNum)` -> sets power for given shield; to primary shield if unspecified.
* `Spring.SetUnitShieldEnabled(unitID, bool enabled, number? weaponNum)` -> sets whether given shield is enabled; primary if unspecified.

Start by locating the existing Spring.SetUnitShieldState entry point and trace how its overloads reach shield state handling. Add the separate Spring.SetUnitShieldPower and Spring.SetUnitShieldEnabled entry points while retaining the existing interface for compatibility; done means both primary and optional-weapon forms are supported.

Written by the indexing model from the issue text.