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/help

@@ -21,14 +21,14 @@ cat << END
   --datarootdir=DIR    default: "\$prefix/share"
   --mandir=DIR         default: "\$datarootdir/man"
   --localstatedir=DIR  default: "\$prefix/var"
-  --libstatedir=DIR    default: "\$localstatedir/lib/unit"
+  --statedir=DIR       default: "\$localstatedir/lib/unit"
   --runstatedir=DIR    default: "\$localstatedir/run/unit"
   --logdir=DIR         default: "\$localstatedir/log/unit"
   --tmpdir=DIR         default: "/tmp"
 
   --incdir=DIR         [deprecated] synonym for --includedir
   --modules=DIR        [deprecated] synonym for --modulesdir
-  --state=DIR          [deprecated] synonym for --libstatedir
+  --state=DIR          [deprecated] synonym for --statedir
   --tmp=DIR            [deprecated] synonym for --tmpdir
 
   --pid=FILE           set pid filename, default: "\$runstatedir/unit.pid"