nickserv: grouped nicks (GROUP/GLIST/UNGROUP) and GHOST

A nick can be grouped to an account and then identifies to it; the store
resolves an alias to its account everywhere it authenticates. GHOST (and
RECOVER) rename off a session using a nick you own. Grouped nicks
federate and are dropped with their account.

modules/nickserv/ghost.rs

@@ -0,0 +1,33 @@
+use crate::engine::db::Db;
+use crate::engine::service::{Sender, ServiceCtx};
+use crate::engine::state::Network;
+
+// GHOST/RECOVER <nick> [password]: rename off a session using a nick you own,
+// either by being identified to its account or giving that account's password.
+pub fn handle(me: &str, guest_nick: &str, guest_seq: &mut u32, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, net: &Network, db: &Db) {
+    let Some(&target) = args.get(1) else {
+        ctx.notice(me, from.uid, "Syntax: GHOST <nick> [password]");
+        return;
+    };
+    let Some(account) = db.resolve_account(target).map(str::to_string) else {
+        ctx.notice(me, from.uid, format!("\x02{target}\x02 isn't registered."));
+        return;
+    };
+    let owns = from.account == Some(account.as_str()) || args.get(2).is_some_and(|pw| db.authenticate(target, pw).is_some());
+    if !owns {
+        ctx.notice(me, from.uid, format!("Access denied. Identify to \x02{account}\x02 or give its password."));
+        return;
+    }
+    let Some(ghost) = net.uid_by_nick(target).map(str::to_string) else {
+        ctx.notice(me, from.uid, format!("Nobody is using \x02{target}\x02."));
+        return;
+    };
+    if ghost == from.uid {
+        ctx.notice(me, from.uid, "That's you.");
+        return;
+    }
+    let guest = format!("{guest_nick}{guest_seq}");
+    *guest_seq = guest_seq.wrapping_add(1);
+    ctx.force_nick(&ghost, &guest);
+    ctx.notice(me, from.uid, format!("\x02{target}\x02 has been freed."));
+}