Process.Kill: don't throw for exited processes (dotnet/corefx#36184)

Process.Kill: don't throw for exited processes (dotnet/corefx#36184)

* Process.Kill: don't throw for exited processes

Implements https://github.com/dotnet/corefx/issues/35257

* Fix exception type

* Fix Windows build failure

* Windows: avoid HasExited side-effects, Unix: refactor code a bit

* Unix: fix error code check

* Refactor test to eliminate timeouts

* Remove !HasExited checks before Process.Kill calls

Commit migrated from https://github.com/dotnet/corefx/commit/1a04816bf3c5be25de09c6cfbb4290752583713b