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
yes, `unlimited` does not really exist but is so big it seems unlimited.  The max is defined in `linux/resource.h` as a const, i would not support an `unlimited` value and force the user to be explicit.