cms-sw/root#123: 612 backport of I/O rules and splitting improvements.

[cmsbuild]

A new Pull Request was created by @pcanal (Philippe Canal) for branch cms/v6-12-00-patches/3f31cef.

Pull Request Reference: cms-sw/root#123

@smuzaffar, @mrodozov, @gudrutis can you please review it and eventually sign? Thanks.

[cmsbuild]

Pull request cms-sw/root#123 was updated.
Latest update by Philippe Canal with commit message
In BuildOld, don't make all unique_ptr as having -> in comment.

unique_ptr can be nullptr too sometimes and anyway TStreamerInfo::Build does not make the same restriction.
The mismatch lead to baffling error message like:

Warning in TStreamerInfo::BuildOld: Cannot convert A::h from type: B to type: B, skip element

This solves one of the problems seen in https://sft.its.cern.ch/jira/browse/ROOT-9702.