Use exec, not shell, to run subprograms

[lars-t-hansen]

See #86 and #87 also; full fixes for those makes the following point moot.

Sonar uses the shell command to run subprograms. This forks an intermediate shell with all its baggage that then runs the subprogram for us. This is not really necessary (it adds overhead); we could be using fork+exec or similar functionality to invoke the subprograms directly.