Fix server-side proxy inappropriate quit when met `accept: too many open files` error
# How this happen

When the `frps` have massive concurrent connection establishments(especially for long-lived connections, e.g. http2, WebSocket), `frps` will use many file descriptors(expected behaviours), and eventually reach the `ulimit -n` limit.

After that the `frps` proxy listener simply log and quit, and the whole `frps` is refused to accepting any further connections.

Which is prone to DDOS attacks.

# How to reproduce this bug

## `frps`
No config file required, just use the defaults.

## `frpc`

### `client.ini`

```ini
[common]
server_addr = 127.0.0.1
server_port = 7000

[too-many-open-files-test]
type = tcp
remote_port = 1053
# Change to yours
local_ip = 192.168.200.1
local_port = 53
```

Why use DNS for test: simply because it's relative long-lived, anything else would be ok.

## `concurrent-conn.sh`

```bash
#!/bin/bash

while :; do
    # Fire and forget(reach `ulimit -n` quickly)
    dig @127.0.0.1 -p1053 dns.google +tcp +short > /dev/null &
done
```
# Reproduce procedures

Env: Ubuntu 20.04.2 LTS x86_64

* `./frps`
* `./frpc -c client.ini`
* `prlimit --pid $(pidof frps) --nofile=128:128`
Explicitly decrease nofile limit of `frps` to reach `ulimit -n` quickly
* `prlimit --pid $(pidof frpc) --nofile=102400:102400`
Explicitly increase nofile limit of `frpc` so `frpc` won't be the bottleneck
* `./concurrent-conn.sh`

# `frps` output

```
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:46129]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:50013]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:40547]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:42587]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:54785]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:49519]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:60873]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:42663]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:51231]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:58995]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:48041]
2021/07/02 20:25:05 [I] [proxy.go:165] [fad8fd4213b57280] [too-many-open-files-test] get a user connection [127.0.0.1:43217]
2021/07/02 20:25:05 [W] [proxy.go:158] [fad8fd4213b57280] [too-many-open-files-test] listener is closed: *net.OpError accept tcp [::]:1053: accept4: too many open files

```

# Verification taken from `concurrent-conn.sh`

> After that the `frps` proxy listener simply log and quit, and the whole `frps` is refused to accepting any further connections.

```
# Check after we met 'accept: too many open files'
$ dig @127.0.0.1 -p1053 dns.google +tcp +short

; <<>> DiG 9.16.1-Ubuntu <<>> @127.0.0.1 -p1053 dns.google +tcp +short
; (1 server found)
;; global options: +cmd
;; connection timed out; no servers could be reached

```
conf/systemd/frpc.service
[Unit]
Description=Frp Client Service
After=network.target

[Service]
Type=simple
User=nobody
Restart=on-failure
RestartSec=5s
ExecStart=/usr/bin/frpc -c /etc/frp/frpc.ini
ExecReload=/usr/bin/frpc reload -c /etc/frp/frpc.ini

[Install]
WantedBy=multi-user.target

[Unit]
Description=Frp Client Service
After=network.target

[Service]
Type=simple
User=nobody
Restart=on-failure
RestartSec=5s
ExecStart=/usr/bin/frpc -c /etc/frp/frpc.ini
ExecReload=/usr/bin/frpc reload -c /etc/frp/frpc.ini
LimitNOFILE=1048576

[Install]
WantedBy=multi-user.target

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