修复unix下日志客户端ip为空，添加配置项设置从哪个header获取

[WeidiDeng]

cloudreve监听unix时日志中客户端ip为空，添加选择项从哪个header获取，默认为"X-Forwarded-For"

[codecov]

Codecov Report

Merging #1314 (545c3b2) into master (fb56b27) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1314      +/-   ##
==========================================
+ Coverage   89.33%   89.42%   +0.08%     
==========================================
  Files          95       95              
  Lines        8273     8273              
==========================================
+ Hits         7391     7398       +7     
+ Misses        721      716       -5     
+ Partials      161      159       -2     

Impacted Files	Coverage Δ
pkg/conf/conf.go	72.72% <ø> (ø)
pkg/filesystem/upload.go	69.00% <0.00%> (-2.93%)	⬇️
pkg/mq/mq.go	95.58% <0.00%> (+1.47%)	⬆️
pkg/cluster/pool.go	94.82% <0.00%> (+9.48%)	⬆️

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