libct: rm BaseContainer and Container interfaces

The only implementation of these is linuxContainer. It does not make
sense to have an interface with a single implementation, and we do not
foresee other types of containers being added to runc.

Remove BaseContainer and Container interfaces, moving their methods
documentation to linuxContainer.

Rename linuxContainer to Container.

Adopt users from using interface to using struct.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/process_linux.go

@@ -300,7 +300,7 @@ type initProcess struct {
 	config          *initConfig
 	manager         cgroups.Manager
 	intelRdtManager *intelrdt.Manager
-	container       *linuxContainer
+	container       *Container
 	fds             []string
 	process         *Process
 	bootstrapData   io.Reader