Renamed --libstatedir to --statedir.

In BSD systems, it's usually </var/db> or some other dir under </var>
that is not </var/lib>, so $statedir is a more generic name.  See
hier(7).

Reported-by: Andrei Zeliankou <zelenkov@nginx.com>
Reported-by: Zhidao Hong <z.hong@f5.com>
Reviewed-by: Konstantin Pavlov <thresh@nginx.com>
Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
Cc: Liam Crilly <liam@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

auto/summary

@@ -13,7 +13,7 @@ Unit configuration summary:
   include directory: ......... "$NXT_INCLUDEDIR"
   man pages directory: ....... "$NXT_MANDIR"
   modules directory: ......... "$NXT_MODULESDIR"
-  state directory: ........... "$NXT_LIBSTATEDIR"
+  state directory: ........... "$NXT_STATEDIR"
   tmp directory: ............. "$NXT_TMPDIR"
 
   pid file: .................. "$NXT_PID"