Testsuite: workaround for Zsh bug with `set -x' a.k.a. `xtrace'

[Stefano Lattarini]

Hello automakers.

I hope this continuous tweaking and reworking of patches doesn't upset you,
but I've recently extended my patch regarding the zsh-related `set -x' bug:
  
  1. I added to the top-level Makefile.am a new maintainer-specific check
      (`sc_tests_stderr_redirection'), which looks for potentially problematic
      stderr redirections in test scripts.
  
  2. I subtituted calls to `AUTOMAKE_run 0' and `AUTOMAKE_fails' respectively
      with calls to `run_command $AUTOMAKE' and `run_command -e 1 $AUTOMAKE';
      I also updated HACKING/README files and syntax check rules accordingly.

While I was at it, I also rebased my previous patch against the latest
`origin/master' commit.

I will soon post the old (amended) patch and the two new ones, in three
distinct mails.

Regards,
     Stefano