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 #20253]

All the way down to Ruby 1.9, `Proc`, `Method`, `UnboundMethod`
and `Binding` always had their own specific clone and dup routine.

This caused various discrepancies with how other objects behave
on `dup` and `clone. [Bug #20250], [Bug #20253].

This commit get rid of `CLONESETUP` and use the the same codepath
as all other types, so ensure consistency.

NB: It's still not accepting the `freeze` keyword argument on `clone`.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

Previously, Kernel#lambda returned a non-lambda proc when given a
non-literal block and issued a warning under the `:deprecated` category.
With this change, Kernel#lambda will always return a lambda proc, if it
returns without raising.

Due to interactions with block passing optimizations, we previously had
two separate code paths for detecting whether Kernel#lambda got a
literal block. This change allows us to remove one path, the hack done
with rb_control_frame_t::block_code introduced in 85a337f for supporting
situations where Kernel#lambda returned a non-lambda proc.

[Feature #19777]

Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>