理解 process.initgroups(user, extra_group)

这个函数是对 linux  C函数 initgroups()  的包装 node.js 官方文档很含糊，还是看 linux C函数文档的解释！很清楚明白。

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.

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