`fiber_join`, `fiber_join_timeout` and `fiber_set_joinable`

Related dev. issue(s): https://github.com/tarantool/tarantool/issues/7562

Product: Tarantool
Since: 3.0
Root document: https://www.tarantool.io/en/doc/latest/dev_guide/reference_capi/fiber/
SME: @ mkostoevr

Note: looks like a bug fix that doesn't require significant documentation changes.

Details

fiber_join, fiber_join_timeout, and fiber_set_joinable behave differently now.

fiber_join and fiber_join_timeout now panic in case if double
join of the given fiber is detected.

fiber_set_joinable now panics if the given fiber is dead or is
joined already. This prevents some amount of error conditions that
could happen when using the API in an unexpected way, including:

• Making a dead joinable fiber non-joinable could lead to a memory
  leak: one can't join the fiber anymore.
• Making a dead joinable fiber joinable again is a sign of attempt
  to join the fiber later. That means the fiber struct may be joined
  later, when it's been recycled and reused. This could lead to a
  very hard to debug double join.
• Making an alive joined fiber non-joinable would lead to the double
  free: once on the fiber function finish, and secondly in the active
  fiber join finish. Risks of making it joinable are described above.
• Making a dead and recycled fiber joinable allowed to join the fiber
  once again leading to a double free.

Any given by the API struct fiber should only be joined once. If a
fiber is joined after the first join on it has finished the behavior
is undefined: it can either be a panic or an incidental join to a
totally foreign fiber.
Requested by @ mkostoevr in https://github.com/tarantool/tarantool/commit/44401529a2253cbf00583a3680ae01f0cb74693c.

Start with the fiber C API root document and locate the entries for fiber_join, fiber_join_timeout, and fiber_set_joinable. Compare their descriptions with the panic conditions, single-join rule, and undefined behavior listed here; the documentation is done when it accurately reflects the current API behavior.