BotServ: add SAY and ACT

The channel's assigned bot can now speak: SAY <#channel> <text> and ACT
<#channel> <text> (CTCP ACTION), gated on channel-operator access. Bots
are tracked in the network view so uid_by_nick resolves them, and the
message is sourced from the bot's own uid via a new ServiceCtx::privmsg.

api/src/lib.rs

@@ -178,6 +178,16 @@ impl ServiceCtx {
         });
     }
 
+    // A channel/target message sourced from one of our pseudo-clients (e.g. a
+    // bot answering a fantasy command, or BotServ SAY).
+    pub fn privmsg(&mut self, from: &str, to: &str, text: impl Into<String>) {
+        self.actions.push(NetAction::Privmsg {
+            from: from.to_string(),
+            to: to.to_string(),
+            text: text.into(),
+        });
+    }
+
     // Hand a registration to the engine to finish: its password derivation runs
     // off the reactor, then the engine commits it and answers `reply`.
     pub fn defer_register(&mut self, account: impl Into<String>, password: impl Into<String>, email: Option<String>, reply: RegReply) {