The freezer controller distinguishes between freezing and frozen [1].
We've had a "Pausing" since dbb515f (Update api proposal,
2014-07-08), but had never implemented it.  This commit adds the
missing implementation.

I haven't updated libcontainer/cgroups, which has used a blocking
freeze change since 3e8849f (implement wait on freeze, 2014-06-04).
The cgroups interface doesn't distinguish between blocking and
non-blocking sets, so there's no way to say "I want to start freezing,
but don't need to block until completion".

I haven't added integration tests either.  All of our current tests
(state.bats and pause.bats) assume a complete transition to 'paused'.
That makes sense because 'runc pause ...' is based on the blocking
libcontainer/cgroups implementation.  The only way to see 'pausing'
would be to have a parallel state call during the pause call.  Here's
how the timing could work out:

1. Call 'pause'.
2. Call 'state', which reads THAWED and outputs 'running' (or
   'created', or whatever).
3. 'pause' starts freezing the controller.
4. Call 'state', which reads FREEZING and outputs 'pausing'.
5. 'pause' finishes freezing the controller.
6. Call 'state', which reads FROZEN and outputs 'paused'.
7. 'pause' exits.

I'm not sure how to get a test where the 'state' call consistently
fires at 4 and not at 2 or 6.

[1]: https://www.kernel.org/doc/Documentation/cgroup-v1/freezer-subsystem.txt

Signed-off-by: W. Trevor King <wking@tremily.us>