[ruby-core:84519] [Ruby trunk Feature#14250] Make `$SAFE` process global state and allow to set 0 again

Issue #14250 has been reported by ko1 (Koichi Sasada).

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Feature #14250: Make `$SAFE` process global state and allow to set 0 again
https://bugs.ruby-lang.org/issues/14250

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Target version: 2.6
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`$SAFE > 1` is removed from Ruby 2.3 and there are some opinion to remove `$SAFE` feature ([Feature #5455]).
There are several reason, but the biggest reason I think is nobody use `$SAFE` correctly.

Also current `$SAFE` is thread/proc local information and it hurts performance (we need to restore `$SAFE` information just after returning proc, even if returning by exception).

Matz said `$SAFE == 1` is similar to warning and it is not a security feature, but one of the programming tool we can use to improve our program (`$SAFE == 3` was for sandbox, security feature).

From this perspective, Matz approved us the followings:

* `$SAFE` is process global, not a Proc local state.
* We can set `$SAFE == 0` when `$SAFE == 1`.

I think we can't make big project with the above changes (how to make multi-thread programming with this `$SAFE`?), but `$SAFE` seems for small project (so-called *scripting*). Anyway if nobody use it, no problem on these changes.

I will commit this change soon.
Please try new spec and point out any problem you got.

Thanks,
Koichi




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