ldap: make uid the default account name

I fail to understand how mail became the uidAttribute way back when LDAP
support was introduced, but it was unintentional and clearly a mistake.

The uid attribute is the standard system login name per RFC4519 2.39 and
what we default to going forward.

default.nix

@@ -445,8 +445,11 @@ in
       dovecot = {
         userFilter = mkOption {
           type = types.str;
-          default = "mail=%{user}";
-          example = "(&(objectClass=inetOrgPerson)(mail=%{user}))";
+          default = with cfg.ldap.attributes; "(|(${mail}=%{user})(${username}=%{user}))";
+          defaultText = literalExpression ''
+            with config.mailserver.ldap.attributes; "(|(''${mail}=%{user})(''${username}=%{user}))";
+          '';
+          example = "(|(mail=%{user})(uid=%{user}))";
           description = ''
             LDAP filter used for LMTP delivery from Postfix and post-login
             information construction, like the home directory.
@@ -459,8 +462,13 @@ in
 
         passFilter = mkOption {
           type = types.nullOr types.str;
-          default = "mail=%{user}";
-          example = "(&(objectClass=inetOrgPerson)(mail=%{user}))";
+          default = with cfg.ldap.attributes; "${username}=%{user}";
+          defaultText = lib.literalExpression ''
+            with config.mailserver.ldap.attributes; "''${username}=%{user}";
+          '';
+          example =
+            with cfg.ldap.attributes;
+            "(&(memberOf=cn=mail_users,ou=groups,dc=example,dc=com)(${username}=%{user}))";
           description = ''
             LDAP filter used to restrict which users are eligible to
             authenticate against Dovecot.
@@ -479,7 +487,7 @@ in
           defaultText = lib.literalExpression ''
             with config.mailserver.ldap.attributes; "''${mail}=%s";
           '';
-          example = "(&(objectClass=inetOrgPerson)(mail=%s))";
+          example = "(mail=%s)";
           description = ''
             LDAP filter used to search for an account by mail, where `%s` is a
             substitute for the address in question.