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/admin/socket.rs

@@ -14,6 +14,7 @@ use crate::shutdown::GracefulShutdown;
 use crate::config::ConfigReloadHandle;
 
 #[derive(Debug, thiserror::Error)]
+#[non_exhaustive]
 pub enum AdminSocketError {
     #[error("admin socket disabled (empty path)")]
     Disabled,