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<std::string_view> 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.