Introduce pseudo-clients with proper services user modes

Services and bots were introduced with only +i. Give them the standard services
umodes, configurable via [server] service_modes (default "iHkBT"): invisible,
hideoper, servprotect — unkillable/unkickable by opers, which the ircd accepts
because the services server is U-lined — bot, and block-CTCP.

Verified live: ChanServ now whoises as "is a bot" (+B) and every pseudo-client
still links cleanly, so every mode was accepted.

src/config.rs

@@ -280,6 +280,15 @@ pub struct Server {
     // rather than the generic default.
     #[serde(default)]
     pub service_host: String,
+    // User modes the service pseudo-clients (services + bots) are introduced with.
+    // Default "iHkBT": invisible, hideoper, servprotect (unkillable — needs the
+    // services server U-lined), bot, block-CTCP. Set per the ircd's loaded modules.
+    #[serde(default = "default_service_modes")]
+    pub service_modes: String,
+}
+
+fn default_service_modes() -> String {
+    "iHkBT".to_string()
 }
 
 fn default_protocol() -> u32 {

src/main.rs

@@ -82,6 +82,7 @@ async fn main() -> Result<()> {
         cfg.uplink.password.clone(),
         cfg.server.protocol,
         ts,
+        cfg.server.service_modes.clone(),
     ));
 
     // Bring up the service modules named in [modules] (default: the full