merge revision(s) 5c7af723,d650b176: [Backport #18497]

	Assuming EXIT_SUCCESS equals 0 is not portable

	---
	 test/ruby/test_fiber.rb | 6 +++---
	 1 file changed, 3 insertions(+), 3 deletions(-)

	`rb_fiber_terminate` must not return [Bug #18497]

	In a forked process from a fiber, the fiber becomes the only
	fiber, `fiber_switch` does nothing as there is no other fibers,
	`rb_fiber_terminate` does not terminate the fiber.  In that case,
	reaches the end of `fiber_entry` finaly, which is declared as
	"COROUTINE" and should never return.
	---
	 cont.c                     |  3 ++-
	 eval_intern.h              |  2 +-
	 test/fiber/test_process.rb | 15 +++++++++++++++
	 test/ruby/test_fiber.rb    |  5 +++++
	 4 files changed, 23 insertions(+), 2 deletions(-)