* cygheap.cc: Include security.h.
* grp.cc (internal_getgrent): New function.
(getgroups): Rearranged using `internal_getgrent' and the new
`cygsid' class.
* passwd.cc (internal_getpwent): New function.
* sec_acl.cc: Use new `cygsid' class throughout.
(acl_access): Use `internal_getgrent' instead of `getgrent'.
* sec_helper.cc: Use new `cygsid' class throughout.
(get_id_from_sid): Use `internal_getgrent' instead of `getgrent'.
Use `internal_getpwent' instead of `getpwent'.
* security.cc: Use new `cygsid' class throughout.
* security.h: Move `MAX_SID_LEN' from winsup.h to here.
Add extern declarations for `internal_getgrent' and `internal_getpwent'.
(class cygsid): New class.
* shared.cc (sec_user): Use new `cygsid' class.
* syscalls.cc (seteuid): Try to set owner to user and primary group to
current group in impersonation token before performing impersonation.
(setegid): Try to set primary group in process token to the new group
if ntsec is on.
* uinfo.cc (internal_getlogin): Use new `cygsid' class.
Try to set owner to user and primary group to current group in process
token if the process has been started from a non cygwin process.
(uinfo_init): Set primary group only if the process has been started
from a non cygwin process.
* winsup.h: Move define for `MAX_SID_LEN' to security.h.
