exec-file/exec-env: reset supplementary groups when changing user

[felixfontein]

No description provided.

[sabre1041]

Are we looking to map the supplemental groups for the specified user or reset it so that it only contains the primary group?

Mapping could be accomplished by something similar to the following

	groupIds, _ := user.GroupIds()
	intGroupIds := make([]int, len(groupIds))
	for i, gid := range groupIds {
		intGroupIds[i], _ = strconv.Atoi(gid)
	}

	err = syscall.Setgroups(intGroupIds)
	if err != nil {
		log.Fatal(err)
	}

[felixfontein]

Ah, I missed user.GroupIds(). Yeah, that makes sense. I've changed it slightly so that it falls back to []int{gid} if GroupIds() returns an error.