ws: Add a /ping REST request for checking if cockpit is running

We implement unrestricted CORS on /ping which allows callers
embedding Cockpit to check if it is running on a given server
before trying to embed it.

From a security perspective, there are *many* other ways to check
if Cockpit is running on a particular server from a third-party
website such as loading an image or script from Cockpit. This
does not weaken security, simply enables the correct approach
for doing such a check.

Fixes #602
Closes #743
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>