Buildscripts now require '#!' line (fixes #9).

* Buildscripts are now executed directly, and now have a valid shebang
  line and be executable (chmod +x).

* Removed option '--xtrace' (it was used to pass '-x' to the buildscript
  shell, and since the above change removes the buildscript shell, this
  option no longer has any use).

* Changed meaning of option '--debug' slightly. (It no longer passes the
  '-v' flag to the buildscript shell, but still have the extra verbosity
  effect.)

* Removed test concerned with testing the '-e' flag of the buildscript
  shell.