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>

pkg/docker/template.Dockerfile

@@ -15,7 +15,7 @@ RUN set -ex \
     && CC_OPT="$(DEB_BUILD_MAINT_OPTIONS="hardening=+all,-pie" DEB_CFLAGS_MAINT_APPEND="-Wp,-D_FORTIFY_SOURCE=2 -fPIC" dpkg-buildflags --get CFLAGS)" \
     && LD_OPT="$(DEB_BUILD_MAINT_OPTIONS="hardening=+all,-pie" DEB_LDFLAGS_MAINT_APPEND="-Wl,--as-needed -pie" dpkg-buildflags --get LDFLAGS)" \
     && CONFIGURE_ARGS="--prefix=/usr \
-                --libstatedir=/var/lib/unit \
+                --statedir=/var/lib/unit \
                 --control=unix:/var/run/control.unit.sock \
                 --pid=/var/run/unit.pid \
                 --log=/var/log/unit.log \