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]