Rename Runtime to PlatformRuntime

This renames the runtime interface to PlatformRuntime to denote the
layer at which the runtime is being abstracted.  This should be used to
abstract different platforms that vary greatly and do not have full
compat with OCI based binary runtimes.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

linux/runtime.go

@@ -79,7 +79,7 @@ func init() {
 	})
 }
 
-var _ = (runtime.Runtime)(&Runtime{})
+var _ = (runtime.PlatformRuntime)(&Runtime{})
 
 // Config options for the runtime
 type Config struct {