Proposal: Allow setting ulimits for containers

Add option to set ulimit settings for containers.
Currently the container inherits from the daemon's ulimit settings.

Support for this was added in libcontainer here docker/libcontainer@7ce34f58cbd4884634a0f1f82c4aaab1168ea0de
I'll add a note that `unlimited` is not supported, since it is likely to be expected since `ulimit` too implements it.