Reject configurations where two configured TCP listeners would bind
the same kernel socket. Covers every pair of listen_addr,
grpc_listen_addr, metrics_listen_addr, and tls_letsencrypt_listen
(when HTTP-01 ACME is in use). Each violation renders as a structured
ConfigError naming both YAML keys, the values the operator wrote, and
a hint pointing at the canonical setup.
Closes the symptom in #3227: the misconfiguration is now rejected at
config-load time with a self-explaining error, instead of failing at
runtime with a misleading "address already in use" log because the
ACME challenge listener and the main HTTP listener competed for the
same port inside the same process.
Fixes#3227
Kristoffer Dalby
284a4891d7