Fix util.popen2 for Python 2.3
1. trigger ImportError early, so fallbacks are activated
2. util.popen2 replaces previous usage of os.popen2, not popen2.popen2
mq: heavy rearrangement of qnew to make it recover reliably from errors.
Closes
issue1269.
mq: make qimport -f work properly. Closes
issue1255.
From Bernhard Leiner.