Fix user counting when ungrouping a node group.

Existing code for ungrouping did not correctly handle user counters:

- counter for the group was not decremented
- counters for containing nodes were not incremented

The latter resulted in losing some nodes after orphan cleaning or several save/reload cycles.

The bug did not have destructive consequences until recently,
because it was compensated by another bug (fixed in rBe993667a46c2).

Maniphest Tasks: T74665, T74682

Differential Revision: https://developer.blender.org/D7332

Fix (unreported) critical mistake in Node copying code.

Nice reversed-logic mistake in rB693721cc7e7d.

How this could remain unnoticed for almost one year is fairly
mysterious, this should have basically broke all node tree copying,
would expect such bug to get reported within days, weeks at most...
Probably because that function is not that much used in current code.

Nice reminder also that those bloody nodetrees still need a lot of
cleanup/refactor/simplification when it comes to ID management code.

Reported/fixed as part of D6484, but this really needs its own commit.