Allow IPC namespace to be shared between containers or with the host

Some workloads rely on IPC for communications with other processes.  We
would like to split workloads between two container but still allow them
to communicate though shared IPC.

This patch mimics the --net code to allow --ipc=host to not split off
the IPC Namespace.  ipc=container:CONTAINERID to share ipc between containers

If you share IPC between containers, then you need to make sure SELinux labels
match.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)

docs/sources/reference/run.md

@@ -50,6 +50,7 @@ following options.
  - [Container Identification](#container-identification)
      - [Name (--name)](#name-name)
      - [PID Equivalent](#pid-equivalent)
+ - [IPC Settings](#ipc-settings)
  - [Network Settings](#network-settings)
  - [Clean Up (--rm)](#clean-up-rm)
  - [Runtime Constraints on CPU and Memory](#runtime-constraints-on-cpu-and-memory)
@@ -131,6 +132,22 @@ While not strictly a means of identifying a container, you can specify a version
 image you'd like to run the container with by adding `image[:tag]` to the command. For
 example, `docker run ubuntu:14.04`.
 
+## IPC Settings
+    --ipc=""  : Set the IPC mode for the container,
+                                 'container:<name|id>': reuses another container's IPC namespace
+                                 'host': use the host's IPC namespace inside the container
+By default, all containers have the IPC namespace enabled 
+
+IPC (POSIX/SysV IPC) namespace provides separation of named shared memory segments, semaphores and message queues.  
+
+Shared memory segments are used to accelerate inter-process communication at
+memory speed, rather than through pipes or through the network stack. Shared
+memory is commonly used by databases and custom-built (typically C/OpenMPI, 
+C++/using boost libraries) high performance applications for scientific
+computing and financial services industries. If these types of applications
+are broken into multiple containers, you might need to share the IPC mechanisms
+of the containers.
+
 ## Network settings
 
     --dns=[]         : Set custom dns servers for the container