Fix readable form test, set target and source paths when no bootscript

- When no build.boot script is present, set target path to . if no -t
  option was specified, and source paths to . if no -s option was
  specified.

- Back out default source and target paths from previous commits. Serves
  no purpose really.

- Fix logic problems with the predicate that tested whether a form could
  be round-tripped through the reader and printer (for pods).