Rewrite: There's no need to declare aux_groups as an array

[vmcj]

          Nitpick: there's no need to declare this as an array if only a single group is set, let alone one of length 10. Maybe for clarity an array is still better though:

		gid_t aux_groups[1];
		aux_groups[0] = rungid;
		if ( setgroups(1, aux_groups) ) error(errno,"cannot clear auxiliary groups");

Also, I guess we can just clear all auxiliary groups, since rungid is already set as the main group, so

		if ( setgroups(0, NULL) ) error(errno,"cannot clear auxiliary groups");

would suffice? (Since this is a C system call, I'm not sure the NULL can be replaced by nullptr.

Originally posted by @eldering in #2344 (comment)

[vmcj]

It might be this is already fixed in the original PR but we should still discuss if we want to keep or remove the array.