sd-event: rename sd_event_source_set_name() to sd_event_source_get_name()

To mirror the recent name change of the concept for sd_bus objects,
follow the same logic for sd_event_source objects, too.
sd-event: implicitly set signal event source's descriptions to the signal name
sd-bus: also allow setting descriptions on bus slots
sd-bus: make use of free_and_strdup() where it makes sense
sd-bus,sd-event: unify error handling of object descriptions

a) When getting the description return ENXIO if none is set

b) Allow setting a description to NULL

c) return ECHILD on fork() like for other calls
sd-bus: rename sd_bus_get_server_id() to sd_bus_get_owner_id()

In kdbus a "server id" is mostly a misnomer, as there isn't any "server"
involved anymore. Let's rename this to "owner" id hence, since it is an
ID that is picked by the owner of a bus or direct connection. This
matches nicely the sd_bus_get_owner_creds() call we already have.
update TODO