bus: rename sd_bus_send_with_reply_and_block() to sd_bus_call()

The call is one of the most important ones we expose, where we place
major emphasis on. We should make sure to give it a short, memorable
name.
test: fix minor memory leak in test-event
bus: introduce concept of a "default" event loop per-thread and make use of it everywhere

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()".
bus: introduce concept of a default bus for each thread and make use of it everywhere

We want to emphasize bus connections as per-thread communication
primitives, hence introduce a concept of a per-thread default bus, and
make use of it everywhere.
bus: export utf8 validator calls as pure functions
id128: introduce SD_ID128_STRING_MAX consant for sizing id128 strings
bus: add api to control auto start message flag
bus: detect blocking message calls to our own connection and return ELOOP early
bus: set no_auto_start flag for GetMachineId calls, so that we don't auto-start if we want to know the machine of a bus service
busctl: show machine name of all registered names
bus: beautify bus_message_dump() output a bit
bus: add missing dump.[ch]
bus: test-bus-objects - add missing header
udev: declare some symbols static
uaccess: Add new DRM render nodes
build-sys: Add --disable-networkd option

For GNOME (Continuous), we are unlikely to require or want
systemd-networkd in the near term future; all of the tools and code
are targeting NetworkManager.

The long term story is still an open question of course, but for now,
there's no reason for gnome-continuous to build or ship this.
build-sys: make networkd man pages conditional
build-sys: add a link test for exported symbols

I know that this is a pretty big net to catch some small fish,
but we *do* regularly forget to properly export symbols that
were supposed to be exported.

This time sd_bus_get_current and some renamed symbols are caught.
build-sys: make public libraries depend on .sym files

This way modyfing the .sym file causes a rebuild.
systemd-python: fix booted() and add two functions to docs

For some reason sphinx doesn't want to show inherited C functions.