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]

Basically a redo of https://github.com/ruby/ruby/commit/a1403fb7cbd1fe0df97c932be9814c86081783dc
but respecting the frozen string literal magic comment

Fixes [Bug #21187]

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

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.

[Backport #21402]

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

[Backport #21094]

The message from dlerror is not our concern.