Accept "user <name> on|off" ACL rule

**Describe the bug**
Dragonfly's ACL file parser rejects lines with fewer than 4 whitespace-separated tokens, including minimal lines like `user default off` that Redis and Valkey accept.

**To Reproduce**
Create an ACL file containing:

```
user default off
```

Start Dragonfly with --aclfile=/path/to/file or run ACL LOAD.

**Expected behavior**
Matching Redis/Valkey, the file loads successfully, creating/updating the default user with no additional category/key/pubsub changes beyond.

**Actual behavior**
```
ERR Error loading: /path/users.acl Error materializing acl file
```

**Root cause**
In acl_family.cc, MaterializeFileContents requires at least 4 tokens per line:

```
std::vector cmds = absl::StrSplit(command, ' ', absl::SkipEmpty());
if (!absl::EqualsIgnoreCase(cmds[0], "USER") || cmds.size() < 4) {
return {};
}
```

user default off produces only 3 tokens (USER, default, off), so it's rejected outright.

Start in acl_family.cc at MaterializeFileContents and inspect how ACL file lines are tokenized and validated. Reproduce with an ACL file containing `user default off` using --aclfile or ACL LOAD, then verify that the minimal rule loads successfully without unintended category, key, or pubsub changes.

Written by the indexing model from the issue text.