Added a few enhancements:

- Change directory to the directory containing the script before executing
- Implement priority for cases where there is both an xyz.GET file and an
  xyz/ directory with a _default_.GET inside of it. The higher-level file
  wins.
- Added test cases around the enhancements

This changeset lowers test coverage, by less than 1 percent. This is because
we are adding a line of code in the untestable process fork block.