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

Fixes k-takata/Onigmo#92.

This fix was ported from oniguruma:
https://github.com/kkos/oniguruma/commit/257082dac8c6019198b56324012f0bd1830ff4ba

https://github.com/k-takata/Onigmo/commit/b1a5445fbeba97b3e94a733c2ce11c033453af73

See <https://bugs.ruby-lang.org/issues/19417#note-3>.

https://unicode.org/reports/tr18/#word states word should match join_control chars.

It did not previously:

```ruby
[*0x0..0xD799, *0xE000..0x10FFFF].map { |n| n.chr 'utf-8' } => all_chars
all_chars.grep(/\p{join_control}/) => jc
jc.count # => 2
jc.grep(/\p{word}/).count # => 0
```
[Backport #19417]

---

Backporting note: I regenerated `enc/unicode/15.0.0/name2ctype.h` using
`make update-unicode`.

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.