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(-)

    note that only backported a part of above patch because of incompatibility
    of ruby_2_7.