Add --pidslimit arg to limit max processes

[aboyett]

The Docker API has offered support for a HostConfig.PidsLimit parameter
to the POST /containers/create route since API version 1.23. This
parameter allows limiting the maximum number of simultaneous processes
within the container.

This feature requires Linux kernel 4.3 or later and pids cgroup support.
Docker API 1.23 is known to be supported in Docker Engine 1.11.2 and
later.

This feature was tested by running ./bin.js alpine --pidslimit 10 on
the host and then running the following inside the spawned container:

/ # for i in $(seq 15) ; do
> eval "echo sleep \$i" ; sleep 10 &
> done
sleep 1
sleep 2
sleep 3
sleep 4
sleep 5
sleep 6
sleep 7
sleep 8
sleep 9
/bin/sh: can't fork