configure.in: check for AIX security library, set in AUTH_LIBS macro
  top.mk: add AUTH_LIBS macro to SECURITY_LIBS
  portable.h.in: added HAVE_AIX_SECURITY macro (via autoheader)
  passwd.c: use AIX getuserpw in chk_unix. Also fix logic in chk_unix:
  	getpwnam must always succeed for the given user. It is not a
	fatal error if getspnam returns no result for the user: On
	systems that support /etc/shadow, its usage is optional. The
	same logic applies for AIX, SCO/HP SecureWare, etc.