[Bug #20755] Frozen string should not be writable via IO::Buffer

	[Bug #20752] Slice of readonly `IO::Buffer` also should be readonly

merge revision(s) c5cf4d4e129f64cb69aaf0a829aed068ef1943c4: [Backport #19542]

	Improve behavioural consistency of unallocated (zero length)
	 `IO::Buffer`. (#9532)

	This makes the behaviour of IO::Buffer.new(0) and IO::Buffer.new.slice(0, 0) consistent.

	Fixes https://bugs.ruby-lang.org/issues/19542 and https://bugs.ruby-lang.org/issues/18805.
	---
	 io_buffer.c                 | 14 ++++++--------
	 test/ruby/test_io_buffer.rb | 35 +++++++++++++++++++++++++++++++++++
	 2 files changed, 41 insertions(+), 8 deletions(-)

	Fix Window private file mapping unlink EACCES issue. (#9358)

	* Don't return early.

	* Add missing `mapping` assignment.

	* Make debug logs conditional.
	---
	 io_buffer.c                 | 18 ++++++++++++------
	 test/ruby/test_io_buffer.rb | 32 ++++++++++++++------------------
	 2 files changed, 26 insertions(+), 24 deletions(-)

Run this test separately because something seems remained unreleased
on Windows.

* Avoiding using `Tempfile` which was retaining the file preventing it from unlinking.

* Restore experimental warnings.

* Documentation and code structure improvements.

* Improved validation of flags, clarified documentation of argument handling.

* Remove inconsistent use of `Example:` and add example to `null?`.

* Expose `IO::Buffer#private?` and add test.

* Raise TypeError when IO::Buffer.map argument is neither IO nor implements #fileno

* Use UNREACHABLE_CODE

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

* Use macro for undef check

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

---------

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

String locking with locktmp is not really part of the public API,
and the test relies in a side effect of using it to protect the
buffer. On other implementations without locktmp this does not
fail. Separate into its own test so it can be excluded from public
API expectations.

* Fix `io_buffer_get_string` default length computation.

When an offset bigger than the size is given, the resulting length will be
computed incorrectly. Raise an argument error in this case.

* Validate all arguments.