Fix process not waking up on signals on OpenBSD

	When using UBF_TIMER_PTHREAD (the UBF handler on OpenBSD), the
	timer_pthread_fn function will not signal the main thread with
	SIGVTALRM in cases where timer_pthread is armed before
	consume_communication_pipe is called.  This is because
	consume_communication_pipe will unarm the timer.

	Fix this by checking the return value of consume_communication_pipe.
	If it returns TRUE and the timer_pthread is disarmed, then signal
	the main thread with SIGVTALRM.

	On OpenBSD, this fixes TestThread#test_thread_timer_and_interrupt, and
	fixes hangs in TestProcess#test_execopts_redirect_open_fifo_interrupt_raise
	and TestProcess#test_execopts_redirect_open_fifo_interrupt_print.
	It also fixes the use of Ctrl+C/SIGINT in irb on OpenBSD. It does not
	cause any test failures on Linux when UBF_TIMER_PTHREAD is forced as
	the UBF handler.

	Fixes [Bug #15798]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67725 b2dd03c8-39d4-4d8f-98ff-823fe69b080e