controller.c: Implement ready/health endpoints

controller.c: Implement ready/health endpoints

These endpoints allow an orchestrator like kubernetes to verify the
status of rspamd (https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/)

Current implementation is very minimal.

The health endpoint allows to verify rspamd internally. It could check
for internal configuration and ensure that rspamd itself is healthy
and available.
The ready endpoint signals that rspamd is ready to receive and process
traffic and thus ensures that configured external components are available.
The readiness check could for example test if configured redis servers or
at least one rspamd upstreams is available.