Improved limits on generating infantry for APCs

[SuperStucco]

This adds better guards when generating infantry for transports, to ensure 'large' transporters like the Maxim (I) are limited to a number that is suitable to their capacity.

Fixes #4535.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 10.80%. Comparing base (f5083a8) to head (6cc4a92).
Report is 116 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4816      +/-   ##
============================================
- Coverage     11.24%   10.80%   -0.44%     
+ Complexity     5941     5715     -226     
============================================
  Files           950      950              
  Lines        131306   131307       +1     
  Branches      19139    19138       -1     
============================================
- Hits          14767    14194     -573     
- Misses       115224   115888     +664     
+ Partials       1315     1225      -90     

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