relax Fiber#transfer's restriction

Using Fiber#transfer with Fiber#resume for a same Fiber is
limited (once Fiber#transfer is called for a fiber, the fiber
can not be resumed more). This restriction was introduced to
protect the resume/yield chain, but we realized that it is too much
to protect the chain. Instead of the current restriction, we
introduce some other protections.

(1) can not transfer to the resuming fiber.
(2) can not transfer to the yielding fiber.
(3) can not resume transferred fiber.
(4) can not yield from not-resumed fiber.

[Bug #17221]

Also at the end of a transferred fiber, it had continued on root fiber.
However, if the root fiber resumed a fiber (and that fiber can resumed
another fiber), this behavior also breaks the resume/yield chain.
So at the end of a transferred fiber, switch to the edge of resume
chain from root fiber.
For example, root fiber resumed f1 and f1 resumed f2, transferred to
f3 and f3 terminated, then continue from the fiber f2 (it was continued
from root fiber without this patch).

spec/ruby/shared/fiber/resume.rb

@@ -35,32 +35,11 @@ describe :fiber_resume, shared: true do
     fiber.send(@method)
   end
 
-  it "runs until Fiber.yield" do
-    obj = mock('obj')
-    obj.should_not_receive(:do)
-    fiber = Fiber.new { 1 + 2; Fiber.yield; obj.do }
-    fiber.send(@method)
-  end
-
-  it "resumes from the last call to Fiber.yield on subsequent invocations" do
-    fiber = Fiber.new { Fiber.yield :first; :second }
-    fiber.send(@method).should == :first
-    fiber.send(@method).should == :second
-  end
-
   it "accepts any number of arguments" do
     fiber = Fiber.new { |a| }
     -> { fiber.send(@method, *(1..10).to_a) }.should_not raise_error
   end
 
-  it "sets the block parameters to its arguments on the first invocation" do
-    first = mock('first')
-    first.should_receive(:arg).with(:first).twice
-    fiber = Fiber.new { |arg| first.arg arg; Fiber.yield; first.arg arg; }
-    fiber.send(@method, :first)
-    fiber.send(@method, :second)
-  end
-
   it "raises a FiberError if the Fiber is dead" do
     fiber = Fiber.new { true }
     fiber.send(@method)