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
Is `unlimited` an allowed value for soft or hard limit? I guess it should be, but the docs don't mention it.