Fix some compilation warnings:

Fix some compilation warnings:
 * src/login.c: "dereferencing type-punned pointer will break
   strict-aliasing rules", add a variable indirection: ptr_pam_user.
 * lib/commonio.c: do not initialize the sb stat structure.
 * lib/pwio.c, lib/shadowio.c, lib/sgroupio.c, lib/groupio.c:
   initialize the security context if WITH_SELINUX.
 * lib/nscd.c: The service argument is not const (used in the exec*
   parameters). This matches with the prototype definition.
 * src/groupmems.c: Avoid ++i when i is also used in the same line.
 * src/newusers.c: i is positive every time it is compared. Add
   cast to unsigned int.
 * src/nologin.c: Use a main() prototype with no arguments.
 * libmisc/getdate.y: Initialize the type and value fields of the
   terminating entry for each TABLE.
 * libmisc/tz.c: Use "TZ=CST6CDT" as the default timezone.