test: add test for reply timeout
test: improve reply timeout test
connection: rework timeout logic

No functional change actually, just a code reordering that makes the
function more readable and saves us one level of indentation.
connection: kick off timeout scan for conn_src, not conn_dst

conn_src is the one we added the reply list to, so this one should be
kicked off.

Note that the first invocation of the timeout scanner will unlikely do
any real work anyway. The reason for calling it here is that it will
implicitly calculate a new deadline, and arm the timer.
test: test-timeout: add dirty hack for loop mode
connection: fix error unwinding of kdbus_conn_new()

It's not safe to unref a half-baked connection, so conduct a real error
unwinding in case of errors.
test: test-kdbus-timeout: cleanups, less verbosity
bus: make conn_seq_last atomic64_t

conn->id is now assigned outside of the bus' lock, so an atomic64_t is
nicer.
TODO: update
update RECV ioctl and implement priority queues (ABI break)
kdbus.h: fix comments
fix kerneldoc