**Describe the bug**
The `nngproxy` subcommand of `nanomq_cli` does not work for me due to an argument parsing issue. I have to revert the change in e9fe4b17 for `nng_proxy.c` (i.e. initialize `idx` as 2 again) to make `nanomq_cli nngproxy` work.

**Expected behavior**

(After reverting e9fe4b17 for `nng_proxy.c`)

```
# ./nanomq_cli nngproxy pub0 --mqtt_url mqtt-tcp://127.0.0.1:1883 --dial tcp://localhost:3327 -t foo
unable to connect tcp://localhost:3327 Connection refused!
# that's OK, and it works when I have a corresponding listener

```

**Actual Behavior**

(Without reverting e9fe4b17 for `nng_proxy.c`)
```
# ./nanomq_cli nngproxy pub0 --mqtt_url mqtt-tcp://127.0.0.1:1883 --dial tcp://localhost:3327 -t foo
NNG url is invalid.
```

**To Reproduce**
See command lines above.

**Environment Details**
 - NanoMQ version: 4d750bac25e01d85fb708d6d2b01d365165ab3a0
 - Operating system and version: custom embedded Linux (armv7hf) with kernel 5.7.1 (patched)
 - Compiler and language used: n/a
 - testing scenario: n/a

**Client SDK**
n/a

**Additional context**
n/a
Argument parsing issue for nngproxy
This is a GitHub Issue
repo:nanomq
owner:emqx
Title : Argument parsing issue for nngproxy
Issue date:

--- start body ---
**Describe the bug**
The `nngproxy` subcommand of `nanomq_cli` does not work for me due to an argument parsing issue. I have to revert the change in e9fe4b17 for `nng_proxy.c` (i.e. initialize `idx` as 2 again) to make `nanomq_cli nngproxy` work.

**Expected behavior**

(After reverting e9fe4b17 for `nng_proxy.c`)

```
# ./nanomq_cli nngproxy pub0 --mqtt_url mqtt-tcp://127.0.0.1:1883 --dial tcp://localhost:3327 -t foo
unable to connect tcp://localhost:3327 Connection refused!
# that's OK, and it works when I have a corresponding listener

```

**Actual Behavior**

(Without reverting e9fe4b17 for `nng_proxy.c`)
```
# ./nanomq_cli nngproxy pub0 --mqtt_url mqtt-tcp://127.0.0.1:1883 --dial tcp://localhost:3327 -t foo
NNG url is invalid.
```

**To Reproduce**
See command lines above.

**Environment Details**
 - NanoMQ version: 4d750bac25e01d85fb708d6d2b01d365165ab3a0
 - Operating system and version: custom embedded Linux (armv7hf) with kernel 5.7.1 (patched)
 - Compiler and language used: n/a
 - testing scenario: n/a

**Client SDK**
n/a

**Additional context**
n/a
--- end body ---
