fiber.wakeup() in Lua and fiber_wakeup() in C could lead to a
crash or undefined behaviour when called on the currently running
fiber.

In particular, if after wakeup was started a new fiber in a
blocking way (fiber.create() and fiber_start()) it would crash in
debug build, and lead to unknown results in release.

If after wakeup was made a sleep with non-zero timeout or an
infinite yield (fiber_yield()), the fiber woke up in the same
event loop iteration regardless of any timeout or other wakeups.
It was a spurious wakeup, which is not expected in most of the
places internally.

The patch makes the wakeup nop on the current fiber making it safe
to use anywhere.

Closes #5292
Closes #6043

@TarantoolBot document
Title: fiber.wakeup() in Lua and fiber_wakeup() in C are nop on self
In Lua `fiber.wakeup()` being called on the current fiber does not
do anything, safe to use. The same for `fiber_wakeup()` in C.