core: Internally deprecate pa_port_available_t to use pa_available_t

core: Internally deprecate pa_port_available_t to use pa_available_t

Generalize the availability flag in order to be used beyond the scope of
ports.

However, pa_port_availability_t is left unchanged to avoid modifying the
protocol and the client API. This should be replaced by pa_available_t
after a validation phase of this new generic enum type.