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
It would not be supported since "unlimited" isn't a real setting.
I suppose we could handle that, but it would make the parsing a little trickier since we'd have to make the flag handle strings instead of ints.