Remove dead_code annotations and add #[non_exhaustive] to public enums

All #[allow(dead_code)] annotations on now-used items have been removed
(acceptor.rs, acme.rs, config.rs, static_config.rs). #[non_exhaustive]
added to TlsMode, ProxyError, AdminSocketError, and ValidationError
with wildcard match arms in main.rs for the non-exhaustive enums.

src/config/validation.rs

@@ -7,6 +7,7 @@ use super::dynamic_config::DynamicConfig;
 use super::static_config::StaticConfig;
 
 #[derive(Debug, Error)]
+#[non_exhaustive]
 pub enum ValidationError {
     #[error("at least one listener must be defined")]
     NoListeners,