sd-ipv4ll: rework callbacks

Firstly, no longer distinguish between STOP and INIT states.

Secondly, do not trigger STOP events when calls to sd_ipv4ll_*() fail. The
caller is the one who would receive the event and will already know that the
call to sd_ipv4ll_*() has failed, so it is redundant.

STOP events will now only be triggered by calling sd_ipv4ll_stop() explicitly
or by some internal error in the library triggered by receiving a packet or
an expiring timeout (i.e., any error that would otherwise not be reported
back to the consumer of the library).

Lastly, follow CODING_STYLE and always return NULL on unref. Protect from
objects being destroyed in callbacks accordingly.