init.sh: disqualify shells for which set -x corrupts stderr

init.sh: disqualify shells for which set -x corrupts stderr

* tests/init.sh: Add a test to disqualify /bin/sh from SunOS 5.11
and OpenBSD 4.7.  They make it so with "set -x", environment settings
appear in stderr output.  For example, this command:
    /bin/sh -c 'set -x; P=1 true 2> err' 2>/dev/null; cat err
prints "P=1" on those two systems: