Add HostServ: virtual hosts

New HostServ pseudo-service assigns and applies vhosts. A vhost is a typed
Option<Vhost>{host,setter,ts} on the account (VhostSet/VhostDeleted events),
applied to the displayed host on identify and re-applied by ON; OFF restores
the normal host for the session. Operators SET/DEL (applied at once to
online sessions) and LIST; hosts are validated as dot-separated labels.

New SetHost NetAction -> InspIRCd ENCAP <target-sid> CHGHOST <uid> <host>.

hostserv/src/list.rs

@@ -0,0 +1,17 @@
+use fedserv_api::{Sender, ServiceCtx, Store};
+
+// LIST: every account with an assigned vhost. Operators only.
+pub fn handle(me: &str, from: &Sender, ctx: &mut ServiceCtx, db: &dyn Store) {
+    if !super::require_oper(me, from, ctx) {
+        return;
+    }
+    let vhosts = db.vhosts();
+    if vhosts.is_empty() {
+        ctx.notice(me, from.uid, "No vhosts have been assigned.");
+        return;
+    }
+    ctx.notice(me, from.uid, format!("Assigned vhosts ({}):", vhosts.len()));
+    for v in &vhosts {
+        ctx.notice(me, from.uid, format!("  \x02{}\x02 — {} (by {})", v.account, v.host, v.setter));
+    }
+}