Create "check" facility to validate environment

[jodh-intel]

The clear-linux-check-config.sh script [1] allows a user to run it with the container argument to determine if their host system is capable of running a Clear Container.

However, that script needs to be manually downloaded.

We should either add a script to the repo (and to EXTRA_DIST) that just includes the CC tests, or add a new check command to the runtime itself that provides the same level of checking.

Additional to the checks already done by [1], we should add:

• Check for KSM (/sys/kernel/mm/ksm/run == 1) (recommended).
• Check for vhost and vhost_net kernel modules (required).

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[1] - https://download.clearlinux.org/current/clear-linux-check-config.sh