Try to emphasize a bit that there should be a mapping between event
loops and threads, hence introduce a logic that there's one "default"
event loop for each thread, that can be queried via
"sd_event_default()".
The sd-event APIs should be available only as part of libsystemd-bus so
that the utility calls are not linked into each independently and we can
minimize the number of libraries we have.
Old static libsystemd-bus.la becomes libsystemd-bus-internal.la.
memfd functions are also exported in the same library.
(Best viewed with --color-words -U0).