理解 process.initgroups(user, extra_group)

这个函数是对 linux  C函数 initgroups()  的包装 node.js 官方文档非常含糊，还是看 linux C函数文档的解释！非常清楚明确。

The initgroups() function uses the getgrouplist(3) function to calculate
     the supplementary group IDs for the user specified in name. This group
     list then set up for the current process using setgroups(2).  The basegid
     is automatically included in the group list.  Typically this value is
     given as the group number from the password file.

參考: http://www.man7.org/linux/man-pages/man3/initgroups.3.html