Environment variables can now be used in bug.cfg.
Fixed linux environment variables.

Linix environment variables now exported, and run script just runs the
command rather than exec-ing it.

These changes allow the environment variables to be seen by the program
being run at run-time as well as at load-time.
Limit include/import by simple name to non-D source files.

Since D always imports with full package ancestry, the ability to
include by a project-unique filename is not appropriate for D.
Fixed a problem in an example Bubfile.

This must have started failing when recent fixes to dependency
checking went in, and I failed to re-test the example.