Refactor runtime and shim to enable integration at the shim level. This would split some of the logic in the shim (process management, etc) to support more generic uses and enable other runtimes to integrate at the shim API level.

This would involve the following:

• Move linux to runtime/linux as it's part of the runtime
• Create a shim v2 API that removes some of the platform specific configuration (keeping v1 for backward compat)
• Move generic packages (proc, etc) to enable sharing among runtimes

Refs: #2250