Revert back to /var/run at a couple of problems

This partially reverts 41a55c46ab

Some specifications we want to stay compatibility actually document
/var/run, not /run, and we should stay compatible with that. In order to
make sure our D-Bus implementation works on any system, regardless if
running systemd or not, we should always use /var/run which is the
only path mandated by the D-Bus spec.

Similar, glibc hardcodes the utmp location to /var/run, and this is
exposed in _UTMP_PATH in limits.h, hence let's stay in sync with this
public API, too.

We simply do not support systems where /var/run is not a symlink → /run.
Hence both are equivalent. Staying compatible with upstream
specifications hence weighs more than cleaning up superficial
appearance.

(cherry picked from commit df1e020461)

man/runlevel.xml

@@ -124,7 +124,7 @@
 
                 <variablelist>
                         <varlistentry>
-                                <term><filename>/run/utmp</filename></term>
+                                <term><filename>/var/run/utmp</filename></term>
 
                                 <listitem><para>The utmp database
                                 <command>runlevel</command> reads the

src/libsystemd/sd-bus/sd-bus.c

@@ -794,8 +794,8 @@ static int parse_container_unix_address(sd_bus *b, const char **p, char **guid)
         machine = NULL;
 
         b->sockaddr.un.sun_family = AF_UNIX;
-        strncpy(b->sockaddr.un.sun_path, "/run/dbus/system_bus_socket", sizeof(b->sockaddr.un.sun_path));
-        b->sockaddr_size = offsetof(struct sockaddr_un, sun_path) + strlen("/run/dbus/system_bus_socket");
+        strncpy(b->sockaddr.un.sun_path, "/var/run/dbus/system_bus_socket", sizeof(b->sockaddr.un.sun_path));
+        b->sockaddr_size = offsetof(struct sockaddr_un, sun_path) + strlen("/var/run/dbus/system_bus_socket");
 
         return 0;
 }