- Save diff output for {stdout,stderr}.diff [feature karen]

  Now we have diff output first, then the error. This often messes up emacs
  error navigation. Also, not atomic.

  So instead we should save the output to a .diff file (could be useful for
  later examination), print the error (including an info line with where is
  the diff), and then (a fragment of) the diff. Theoretically, the .diff
  can be large, so probably makes sense to limit it to a few KB?

- Print signal/core dump like shell/make [feature karen]

  Somehow bash and GNU make can detect which signal caused abnormal
  termination and print it as well as whether core was dumped, for example:

  Aborted (core dumped)
  Segmentation fault

  Would be nice to be just as smart plus, in case of a core, also print where
  it is (normally terminated executable's working directory).

- Tests cleanup [quality karen]

  Need to resolve the following issues in testscript used to test build2:

  * add standard file headers
  * assign ids to all tests
  * @@-issues
  * conformance to testscript style guide
  * &&-expression fix for cat & build
  * use test.{redirect,cleanup} where appropriate
  * cross-testing fixes and conversions