cli设置frps的log_file无效，日志会打印在屏幕中

[status301]

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Use the commands below to provide key information from your environment:
You do NOT have to include this information if this is a FEATURE REQUEST

What version of frp are you using (./frpc -v or ./frps -v)?
[root@hk frp_0.25.0_linux_amd64]# ./frps -v
0.25.0
[root@hk frp_0.25.0_linux_amd64]#

What operating system and processor architecture are you using (go env)?
[root@hk frp_0.25.0_linux_amd64]# go env
GOARCH="amd64"
GOHOSTARCH="amd64"
GOTOOLDIR="/usr/lib/golang/pkg/tool/linux_amd64"

Configures you used:
cli:
[root@hk frp_0.25.0_linux_amd64]# ./frps --bind_port 65401 --kcp_bind_port 65402 --vhost_http_port 65403 --vhost_https_port 65404 --bind_udp_port 65405 --token Frp-Token --dashboard_port 65406 --dashboard_user Frp-Token --dashboard_pwd Frp-Token --log_file ./frps.log --log_level trace --log_max_days 3

Steps to reproduce the issue:

1. ./frps --bind_port 65401 --kcp_bind_port 65402 --vhost_http_port 65403 --vhost_https_port 65404 --bind_udp_port 65405 --token Frp-Token --dashboard_port 65406 --dashboard_user Frp-Token --dashboard_pwd Frp-Token --log_file ./frps.log --log_level trace --log_max_days 3

Describe the results you received:
没有将日志存入frps.log，而是打印在屏幕上的
[root@hk frp_0.25.0_linux_amd64]# ./frps --bind_port 65401 --kcp_bind_port 65402 --vhost_http_port 65403 --vhost_https_port 65404 --bind_udp_port 65405 --token Frp-Token --dashboard_port 65406 --dashboard_user Frp-Token --dashboard_pwd Frp-Token --log_file ./frps.log --log_level trace --log_max_days 3
2019/03/11 23:08:05 [I] [service.go:136] frps tcp listen on 0.0.0.0:65401
2019/03/11 23:08:05 [I] [service.go:145] frps kcp listen on udp 0.0.0.0:65402
2019/03/11 23:08:05 [I] [service.go:178] http service listen on 0.0.0.0:65403
2019/03/11 23:08:05 [I] [service.go:199] https service listen on 0.0.0.0:65404
2019/03/11 23:08:05 [I] [service.go:218] nat hole udp service listen on 0.0.0.0:65405
2019/03/11 23:08:05 [I] [service.go:229] Dashboard listen on 0.0.0.0:65406
2019/03/11 23:08:05 [I] [root.go:204] Start frps success
^C
[root@hk frp_0.25.0_linux_amd64]

Describe the results you expected:
--log_file ./frps.log --log_level trace --log_max_days 3

Additional information you deem important (e.g. issue happens only occasionally):
可以使用 >>frp.log

Can you point out what caused this issue (optional)