All tests pass now under WSL2. They should pass under WSL1 too, but only
WSL2 is integrated into the test fleet right now. This change also fills
in some gaps in the error numbers.
Fixes#665
This change reduces the .bss memory requirement for all executables by
O(64kb). The brk system calls are now fully tested and figured out and
might be useful for tiny programs that only target System Five.