vsftpd上传文件权限配置规则
折腾了老半天才搞清楚规则。在vsftpd.conf内，主要两个配置项相关，运行man vsftpd.conf可以看到：
file_open_mode
The permissions with which uploaded files are created. Umasks
are applied on top of this value. You may wish to change to 0777
if you want uploaded files to be executable.
Default: 0666
local_umask
The value that the umask for file creation is set to for local
users. NOTE! If you want to specify octal values, remember the
"0" prefix otherwise the value will be treated as a base 10
integer!
Default: 077
我需要上传后的文件权限为755，则应该这么配置：
file_open_mode=0755
local_unmask=022 （这个配置表示从file_open_mode赋予的权限中去掉22，即owner group和others的写权限）
