Fix handling of `error`/`errno` in `io_internal_wait`. (#12961)

	[Bug #21195]

	Ensure IO.copy_stream buffer is an independent string

	Otherwise, changes to the buffer by the destination write method
	could result in data changing for supposedly independent strings.

	Fixes [Bug #21131]

	[Bug #20924] Fix reading with delimiter in wide character encodings

	[Bug #20787] Check the separator in `IO#readline` as well as 3.2

This commit moves IO#readline to Ruby.  In order to call C functions,
keyword arguments must be converted to hashes.  Prior to this commit,
code like `io.readline(chomp: true)` would allocate a hash.  This
commits moves the keyword "denaturing" to Ruby, allowing us to send
positional arguments to the C API and avoiding the hash allocation.

Here is an allocation benchmark for the method:

```
x = GC.stat(:total_allocated_objects)
File.open("/usr/share/dict/words") do |f|
  f.readline(chomp: true) until f.eof?
end
p ALLOCATIONS: GC.stat(:total_allocated_objects) - x
```

Before this commit, the output was this:

```
$ make run
./miniruby -I./lib -I. -I.ext/common  -r./arm64-darwin22-fake  ./test.rb
{:ALLOCATIONS=>707939}
```

Now it is this:

```
$ make run
./miniruby -I./lib -I. -I.ext/common  -r./arm64-darwin22-fake  ./test.rb
{:ALLOCATIONS=>471962}
```

[Bug #19890] [ruby-core:114803]

Deprecate Kernel#open and IO support for subprocess creation and
forking. This deprecates subprocess creation and forking in

- Kernel#open
- URI.open
- IO.binread
- IO.foreach
- IO.readlines
- IO.read
- IO.write

This behavior is slated to be removed in Ruby 4.0

[Feature #19630]

* Documentation consistency.

* Improve consistency of `pread`/`pwrite` implementation when given length.

* Remove HAVE_PREAD / HAVE_PWRITE - it is no longer optional.

This test should be fixed and fast now because the closing thread sleeps
appropriately waiting for the file descriptor to be unused.

Fixes [Bug #19380]