3e5ce23c99 introduced a regression in case the freed Main was part of a
list, and was supposed to be removed from it, since calling
`BLI_remlink` does _not_ clear the `prev`/`next` pointers of the removed
link.
This commit also contains a few more tweaks to recent related b3f42d8e98
commit.
While this behavior can be useful in some cases, it can also create
issues (as in one of own recent commits, 3e5ce23c99), since it
implicetly keeps the removed linknode 'linked' to the listbase.
At least warn about it in the documentation of `BLI_remlink`.
This commit introduces a new Main boolean flag that marks is as invalid.
Higher-level file reading code does checks on this flag to abort reading
process if needed.
This is an implementation of the #105083 design task.
Given the extense of the change, I do not think this should be
considered for 3.5 and previous LTS releases.
Bastien Montagne