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/lib.rs

@@ -0,0 +1,70 @@
+//! HostServ assigns virtual hosts (vhosts) to accounts and applies them to the
+//! displayed host. Members toggle their own with ON/OFF; operators assign them
+//! with SET/DEL and review with LIST. `lib.rs` holds the dispatcher; each
+//! command lives in its own file.
+
+use fedserv_api::{NetView, Priv, Sender, Service, ServiceCtx, Store};
+
+#[path = "on.rs"]
+mod on;
+#[path = "off.rs"]
+mod off;
+#[path = "set.rs"]
+mod set;
+#[path = "del.rs"]
+mod del;
+#[path = "list.rs"]
+mod list;
+
+pub struct HostServ {
+    pub uid: String,
+}
+
+impl Service for HostServ {
+    fn nick(&self) -> &str {
+        "HostServ"
+    }
+    fn uid(&self) -> &str {
+        &self.uid
+    }
+    fn gecos(&self) -> &str {
+        "Host Services"
+    }
+
+    fn on_command(&mut self, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, net: &dyn NetView, db: &mut dyn Store) {
+        let me = self.uid.as_str();
+        match args.first().map(|s| s.to_ascii_uppercase()).as_deref() {
+            Some("ON") => on::handle(me, from, ctx, db),
+            Some("OFF") => off::handle(me, from, ctx, net, db),
+            Some("SET") => set::handle(me, from, args, ctx, net, db),
+            Some("DEL") => del::handle(me, from, args, ctx, net, db),
+            Some("LIST") => list::handle(me, from, ctx, db),
+            Some("HELP") | None => ctx.notice(me, from.uid, "HostServ gives you a vhost: \x02ON\x02 activates your assigned vhost, \x02OFF\x02 restores your normal host. Operators use \x02SET\x02 <account> <host>, \x02DEL\x02 <account> and \x02LIST\x02."),
+            Some(other) => ctx.notice(me, from.uid, format!("I don't know the command \x02{other}\x02. Try \x02HELP\x02.")),
+        }
+    }
+}
+
+// Operator gate for vhost administration.
+fn require_oper(me: &str, from: &Sender, ctx: &mut ServiceCtx) -> bool {
+    if from.privs.has(Priv::Admin) {
+        return true;
+    }
+    ctx.notice(me, from.uid, "Access denied — assigning vhosts is for services operators.");
+    false
+}
+
+// Whether `host` is a syntactically valid vhost: a hostname of dot-separated
+// labels using letters, digits and hyphens, not too long.
+pub(crate) fn valid_vhost(host: &str) -> bool {
+    if host.is_empty() || host.len() > 64 || !host.contains('.') {
+        return false;
+    }
+    host.split('.').all(|label| {
+        !label.is_empty()
+            && label.len() <= 63
+            && !label.starts_with('-')
+            && !label.ends_with('-')
+            && label.bytes().all(|b| b.is_ascii_alphanumeric() || b == b'-')
+    })
+}