* libmisc/pwdcheck.c: Do not include <pwd.h>. Include <shadow.h>

* libmisc/pwdcheck.c: Do not include <pwd.h>. Include <shadow.h>
  and "pwauth.h" only when compiled without PAM support.
* src/chfn.c, src/chsh.c: Do not include <shadow.h>
* lib/commonio.c: Do not include <shadow.h>. Do not include
  <pwd.h>. Include "nscd.h" instead of <nscd.h>.
* configure.in: Do not check if shadow.h exist, but make sure it
  exists.
* libmisc/pwdcheck.c, src/chfn.c, src/chsh.c, lib/defines.h,
  lib/shadowmem.c, lib/shadowio.c, lib/commonio.c:
  HAVE_SHADOW_H is no more needed (shadow.h should always exist).