last one in the pipe to make sure failed regression tests don't bake a
cookie when REGRESS_LOG is set (default).
Also redirect stderr to regress log and introduce REGRESS_STATUS_IGNORE
variable so post-regress targets like found in cpan.port.mk can do the
actual failure detection.
Problem noticed by bluhm@, latching of exit code suggested by steven@
Testing, feedback and ok on earlier version by bluhm@, 'go for it' espie@