MemoServ: add SET NOTIFY and SET LIMIT

SET NOTIFY {ON|OFF} controls whether you're told about new memos on login;
SET LIMIT <n>|NONE sets your mailbox cap (honoured on SEND, NONE = the
network default). New memo_notify/memo_limit account fields + one
MemoPrefsSet event.

modules/memoserv/src/set.rs

@@ -0,0 +1,38 @@
+use echo_api::{Sender, ServiceCtx, Store};
+
+// SET NOTIFY {ON|OFF} | SET LIMIT <n>|NONE: your MemoServ preferences.
+pub fn handle(me: &str, from: &Sender, account: &str, args: &[&str], ctx: &mut ServiceCtx, db: &mut dyn Store) {
+    match args.get(1).map(|s| s.to_ascii_uppercase()).as_deref() {
+        Some("NOTIFY") => {
+            let on = match args.get(2).map(|s| s.to_ascii_uppercase()).as_deref() {
+                Some("ON") => true,
+                Some("OFF") => false,
+                _ => {
+                    ctx.notice(me, from.uid, "Syntax: SET NOTIFY {ON|OFF}");
+                    return;
+                }
+            };
+            db.set_memo_notify(account, on);
+            ctx.notice(me, from.uid, format!("New-memo notification is now \x02{}\x02.", if on { "on" } else { "off" }));
+        }
+        Some("LIMIT") => {
+            let limit = match args.get(2) {
+                None => None,
+                Some(&n) if n.eq_ignore_ascii_case("NONE") || n.eq_ignore_ascii_case("OFF") => None,
+                Some(&n) => match n.parse::<u32>() {
+                    Ok(v) if v <= super::MAX_MEMOS as u32 => Some(v),
+                    _ => {
+                        ctx.notice(me, from.uid, format!("The limit must be a number from 0 to {}, or NONE.", super::MAX_MEMOS));
+                        return;
+                    }
+                },
+            };
+            db.set_memo_limit(account, limit);
+            match limit {
+                Some(v) => ctx.notice(me, from.uid, format!("Your mailbox limit is now \x02{v}\x02.")),
+                None => ctx.notice(me, from.uid, format!("Your mailbox limit is now the network default (\x02{}\x02).", super::MAX_MEMOS)),
+            }
+        }
+        _ => ctx.notice(me, from.uid, "Syntax: SET NOTIFY {ON|OFF} | SET LIMIT <n>|NONE"),
+    }
+}