[ruby-core:104327] [Ruby master Feature#17994] Clarify `IO.read` behavior and add `File.read` method

Issue #17994 has been reported by mame (Yusuke Endoh).

----------------------------------------
Feature #17994: Clarify `IO.read` behavior and add `File.read` method
https://bugs.ruby-lang.org/issues/17994

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
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`IO.read` creates a subprocess when a given file name starts with a `|` character.

```
irb(main):001:0> IO.read("| ls /etc/passwd")
=> "/etc/passwd\n"
```

To disable this feature, `File.read` can be used.

```
irb(main):002:0> File.read("| ls /etc/passwd")
(irb):2:in `read': No such file or directory @ rb_sysopen - | ls /etc/passwd (Errno::ENOENT)
```

So, as far as I know, `File.read` is more prefereable to `IO.read` if a user want to just read a file.

However, in terms of the implementation, there is no definition of `File.read`. `File.read` invokes `IO.read` because `IO` is a superclass of `File`, and `IO.read` creates a subprocess only when its receiver is exactly the `IO` class.

I think there are two problems in the current situation:

1. The rdoc of `IO.read` does not explain the behavior to disable a subprocess invocation.
2. The rdoc does not have an entry for `File.read`.

I've created a PR to address the two issues by clarifying `IO.read` behavior and defining `File.read` as an alias to `IO.read`.

https://github.com/ruby/ruby/pull/4579



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