* thread.c (rb_threadptr_execute_interrupts_common): check signal
  deliverly if it is main thread.
  fixes [ruby-dev:44005] [Ruby 1.9 - Bug #4950]
* bootstraptest/test_fork.rb: add a test for above.
* signal.c (rb_get_next_signal): skip if signal_buff is empty.
  (check signal_buff.size first)
* vm_core.h: remove unused variable rb_thread_t::exec_signal.
* thread.c (rb_thread_check_trap_pending): check
  rb_signal_buff_size() because rb_thread_t::exec_signal
  is no longer available.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@32345 b2dd03c8-39d4-4d8f-98ff-823fe69b080e