serve: list tuners & feeders during CLI command 'check'

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
serve_client: add function list_clients() for use in the CLI

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
serve: safely reclaim server & tuner recources during check()

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>