(class cygsid): Use cygpsid as base. Remove members psid, get_id,
get_uid, get_gid, string, debug_printf and the == and != operators.
(cygsidlist::clear_supp): Only do work if setgroups has been called.
* sec_helper.cc: Define sid_auth NO_COPY.
(cygpsid::operator==): New operator.
(cygpsid::get_id): New function.
(cygpsid::string): New function.
(cygsid::string): Delete.
(cygsid::get_id): Delete.
* pwdgrp.h: Change arguments of internal_getpwsid,
internal_getgrsid and internal_getgroups to cygpsid.
* passwd.cc (internal_getpwsid): Change argument from cygsid to cygpsid.
* grp.cc (internal_getgrsid): Ditto.
(internal_getgroups): Ditto.
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