(cygwin_select): Move fd_set copying logic from ::wait to here. Use common
return through sell.poll.
(select_stuff::wait): Just return success or failure and let caller fill in
fd_set.
* pinfo.h (pinfo): Eliminate self-referential pointer to sidbuf since pinfo
structure exists at random locations now.
* fork.cc (fork): Use 'use_psid' element to control when the psid is relevant.
* shared.cc (sec_user): Ditto.
* spawn.cc (spawn_guts): Ditto.
* uinfo.cc (internal_getlogin): Ditto.
* syscall.cc (seteuid): Ditto. Set use_psid element.
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