sd-bus: make sd-bus fiber-aware

sd-bus: make sd-bus fiber-aware

Two changes to teach sd-bus how to behave when called from a fiber, in
order of increasing depth:

2. sd_bus_call() now redirects to a new bus_call_suspend() helper when
   the caller is a fiber whose event loop is the same one the bus is
   attached to. The plain bus_poll() path serializes all bus traffic on
   the slot's reply (only one method call can be in flight per
   sd_bus*), which would defeat the point of running multiple fibers
   against one bus. bus_call_suspend() builds on the async sd-bus API:
   it wraps the call in a new BusFuture (sd-bus/bus-future.{c,h}) that
   resolves when the reply or method-error arrives, lets the fiber
   await that future, and surfaces the reply to the caller via
   future_get_bus_reply(). Because the futures live on the event loop
   rather than a per-bus slot, multiple fibers can drive concurrent
   method calls against the same bus.

3. A new private SD_BUS_VTABLE_METHOD_FIBER flag dispatches a vtable
   method handler on its own fiber, so handlers are free to use
   sd_bus_call() against the same bus, sd_fiber_sleep(), loop_read(),
   etc. without stalling the event loop for other connections or
   handlers. The flag stays out of sd-bus-vtable.h (its bit value is
   reserved there to prevent collisions) — the fiber runtime is a
   systemd-internal implementation detail.

Lifecycle of fiber-dispatched handlers is tracked on the bus itself: a
new bus->fiber_futures set holds a ref to each in-flight handler.
bus_enter_closing() cancels every entry and process_closing() returns
with the bus still in CLOSING state until the set drains, so we can be
sure no fiber handler outlives the bus. bus_fiber_resolved() removes
the entry on completion. bus_free()'s assert(set_isempty()) makes the
invariant load-bearing.

Note that plain sd_bus_call() already works correctly on a fiber as it
calls ppoll_usec() which has already been modified to suspend when
running on a fiber.

To exercise these changes the existing thread-based client/server
sd-bus tests (test-bus-chat, test-bus-objects, test-bus-peersockaddr,
test-bus-server, test-bus-watch-bind) are migrated to fibers, and a
new test-bus-fiber is added that covers SD_BUS_VTABLE_METHOD_FIBER —
including handlers that issue nested sd_bus_call() on the same bus, the
cancel-on-close path, and concurrent dispatches across multiple fibers.