Set `current-load-relative-directory' to the path of the module file so
paths that it resolves in the description (in `describe-modeul') are not
bogus.
* a single function to set up all environment parameters.
* improve `getarg's treatment of default thunk
* Add an error display handler that doesn't show the context and instead
add a ,bt command to show it.
Not too much, but already tests large parts of sensitive code. Caught a
bug where ,top would use (enter! #f) but enter grabbed the wrong
namespace since it was instantiated in the wrong namespace.
