Merged the slinkp-datetime-200007 branch: fix the DateTime(anotherDateTime) constructor to preserve timezones.

'unrestrictedTraverse' (apparent NameError).

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for starting application-level threads. 

o Also, use tagged release of ZODB-based externals.

Base the default location of softwarehome on Zope2, which is guaranteed not to be a namespace but always located inside the Zope2 software home. This fixes Zope2 breaking on systems which uses namespace packages for Products.*

Revert c81408 HelpSys changes, it didn't make much of a difference. Making the whole help system optional would be a better approach.

Revert ISO8601 interpretation to previously broken behaviour (assume GMT rather than local time as per spec)

(culprit from the last refactoring)