Convert most of rust_run_program.cpp to rust (issue #2674).
Remove errant trailing whitespace.
Fix some issues with test_destroy_actually_kills:
- it is now cross platform, instead of just unix
- it now avoids sleeping (fixing issue #6156)
- it now calls force_destroy() when force = true (was a bug)
auto merge of #6140 : Dretch/rust/run-with-rust, r=brson

Even more of `core::run` could be rust-ified -- I believe that access to the C extern environ can be done with rust now. I did not do this because some special casing is needed for OSX and I don't have a mac I can test with.

I think this will also fix #6096.