Set CampaignID= when launching missions + don't load multiple Battle*.ini files anymore

[Rampastring]

https://github.com/CnCNet/ts-patches/commit/8632b2dc257b97ac43c66a8b6d0a73cb34165f68

A recent change made to CnCNet ts-patches requires the client to write CampaignID= to spawn.ini when launching singleplayer missions, with the ID depending on the index of the entry in Battle*.ini.

Because handling indexes can get messy with multiple Battle.ini and I haven't seen many mods using multiple Battle.ini files, this PR also makes the client not load multiple Battle.ini files anymore. Only the first one that contains missions is read.

[github-actions[bot]]

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