Add IEnumerable.RandomOrDefault extension.
Improve support for picking a valid actor spawn cell.
Smarter unit spawn logic. Fixes #3496.
Fix Doughnut Hole spawn behavior.
Remove obsolete DefaultStartingUnits parameter.

It was only actually used by bomber-john. Everything
else also removed SpawnMPUnits, making it redundant.
Add lobby support for configuring starting units.
Enable starting units in C&C.
Merge pull request #3550 from pchote/starting-units

Support starting units.