Method#unbind clones the method entry, preserving its defined_class.
For methods mixed in via include/extend, defined_class is an ICLASS,
causing UnboundMethod#== to return false when comparing against the
same method obtained via Module#instance_method.

Resolve ICLASS defined_class in method_eq.

[Bug #21873]

	[PATCH] [Bug #21666] Get rid of use of unspecified values

	ruby2_keywords warnings: Quote non-UTF8 method names fully

	It used to quote only part of the method name because NUL byte in
	the method terminates the C string:

	```
	(irb)> "abcdef".encode("UTF-16LE").bytes
	=> [97, 0, 98, 0, 99, 0, 100, 0, 101, 0, 102, 0]
	```

	```
	expected: /abcdef/
	actual: warning: Skipping set of ruby2_keywords flag for a (method not defined in Ruby)\n".
	```

	Do not respect ruby2_keywords on method/proc with post arguments

	Previously, ruby2_keywords could be used on a method or proc with
	post arguments, but I don't think the behavior is desired:

	```ruby
	def a(*c, **kw) [c, kw] end
	def b(*a, b) a(*a, b) end
	ruby2_keywords(:b)

	b({foo: 1}, bar: 1)
	```

	This changes ruby2_keywords to emit a warning and not set the
	flag on a method/proc with post arguments.

	While here, fix the ruby2_keywords specs for warnings, since they
	weren't testing what they should be testing.  They all warned
	because the method didn't accept a rest argument, not because it
	accepted a keyword or keyword rest argument.

	[Bug #20009] Support marshaling non-ASCII name class/module

	stat command is not provided on Windows

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