Add a kernel parameter to create a trusted sandbox

[mpcjanssen]

Fixes #13

[rmculpepper]

Thanks for the PR! There is one issue, which is that call-with-trusted-sandbox-configuration does not set the parameters listed under "Not set by call/trusted" in the code of call-with-kernel-sandbox-configuration. The most important is sandbox-namespace-specs: without that setting, the picts in the Getting Started example notebook don't render as images.

[mpcjanssen]

@rmculpepper I have created a separate configuration proc for the trusted scenario. Networking still works and images display correctly again.

[mpcjanssen]

@rmculpepper anything remaining to have this merged?

[rmculpepper]

I think this is good. Thanks!