chasquid: Fail at RCPT TO time if a user does not exist

It's more convenient and in line with standard practice to fail RCPT TO if the
user does not exist.

This involves making the server and client aware of aliases, but it doesn't
end up being very convoluted, and simplifies other code.

internal/userdb/userdb.go

@@ -202,6 +202,14 @@ func (db *DB) RemoveUser(name string) bool {
 	return present
 }
 
+// HasUser returns true if the user is present, False otherwise.
+func (db *DB) HasUser(name string) bool {
+	db.mu.Lock()
+	_, present := db.db.Users[name]
+	db.mu.Unlock()
+	return present
+}
+
 ///////////////////////////////////////////////////////////
 // Encryption schemes
 //