set thread name visible with ps command on GNU/Linux.
  Ex. ps -o %c -L

* thread.c (thread_start_func_2): Call native_set_thread_name at
  beginning.
  (rb_thread_inspect_msg): Extract from rb_thread_inspect.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47670 b2dd03c8-39d4-4d8f-98ff-823fe69b080e