avahi: Prepare code for support of renderer discovery

avahi: Prepare code for support of renderer discovery

Instead of passing the services_discovery_t as userdata for the callbacks,
use a struct discovery_sys_t which is common for both the input and renderer
private data structures.

The discovery_sys_t is embedded into the service type specific private
member structs, i.e. struct services_discovery_sys_t (done here) and
struct vlc_renderer_discovery_sys (next patch).

Signed-off-by: Thomas Guillem <thomas@gllm.fr>
(cherry picked from commit da019696fbc76b7afea69b33f2c537c85a350a59)
Signed-off-by: Thomas Guillem <thomas@gllm.fr>
Gbp-Pq: Name 0005-avahi-Prepare-code-for-support-of-renderer-discovery.patch