core: introduce system fiber

There are a number of internal system fibers which are not supposed to
be cancelled.

Let's introduce `FIBER_IS_SYSTEM` flag that will indicate, if the fiber
can be explicitly killed. If this flag is set, killing functions will
just ignore cancellation request.

This commit introduce blocking system fiber cancelling only from the Lua
public API, as it is more important to have it right. The prohibition to
cancel fibers from C API will be introduced later.

Related to #7448
Part of #7473

NO_DOC=internal
NO_TEST=will be added in subsequent commit
NO_CHANGELOG=internal