thread.c: thread_do_start

	* thread.c (thread_do_start): extract from a macro in
	  thread_start_func_2 for debugger.
	thread.c: Thread.start with Symbol

	* thread.c (thread_do_start): fix segfault at start with Symbol.
	  proc created by Symbol#to_proc does not have environment unless
	  using refinements.  [ruby-core:80147] [Bug #13313]
	Fiber also has same issue. [Bug #13313]

	* thread.c (rb_vm_proc_local_ep): added.

	* cont.c (rb_fiber_start): use rb_vm_proc_local_ep().


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@58003 b2dd03c8-39d4-4d8f-98ff-823fe69b080e