libcontainer: remove LinuxFactory

Since LinuxFactory has become the means to specify containers state
top directory (aka --root), and is only used by two methods (Create
and Load), it is easier to pass root to them directly.

Modify all the users and the docs accordingly.

While at it, fix Create and Load docs (those that were originally moved
from the Factory interface docs).

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

libcontainer/restored_process.go

@@ -79,8 +79,8 @@ func (p *restoredProcess) forwardChildLogs() chan error {
 }
 
 // nonChildProcess represents a process where the calling process is not
-// the parent process.  This process is created when a factory loads a container from
-// a persisted state.
+// the parent process. This process is created when Load loads a container
+// from a persisted state.
 type nonChildProcess struct {
 	processPid       int
 	processStartTime uint64