Revert to the standard Zope 3 provider-expression; this is reasonable and required after the sane acquisition branch has been merged.

Protect against non-existing zope.conf path and products directories. This makes it possible to run a Zope instance without a Products or lib/python directory.

follow-up to r92860...unrestrictedTraverse also needs to acquisition-wrap implicit acquirers found via adaptation (fixing regression from merge of philikon-aq branch)

fix regression introduced in merge of philikon-aq branch; implicit acquirers looked up as named adapters still need an acquisition wrapper.  see http://plone.org/support/forums/core#nabble-td1473889|a1479594 and following for discussion.

Add check for doomed transactions in default transaction manager, abort silently if the tm tries to commit a doomed transaction

 - Rather nasty fix to work around Zope 3 exceptions that have more than one positional argument on the constructor. Also, pass 'handle_errors' argument used for functional http testing around and use that on raise_standardErrorMessage to decide if we need to care about rendering a full HTML traceback.

  argument/attribute missing in testbrowser tests.

  provided tests and implementation for guarded_zip (RestrictedPython).