[FS#988] Procd does not respect a user's groups when using parameter user

[LEDE Bugs]

The following task has a new comment added:

FS#988 - Procd does not respect a user's groups when using parameter user
User who did this - Yousong Zhou (yousong)

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The issue should be that procd only sets effective group id to user's primary group id while Alexander the reporter expects it also sets supplementary group ids for instance processes ;)  I think this is a valid feature request, better than adding another "group" option.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=988#comment4303