merge with pkg-update: fix #661248
- initscript is capable of handling situations when /var/run is on tmpfs
(/var/run/luci is re-created every time it is not present when
starting the luci service), hinted by Fabio Di Nitto
- there is a KEEP_RUNTIME_DATA var in initscript which if set to 0
(the default) will remove all transient data (cache and sessions
directories only, not a pid file) both when the service is stopped
and when the service is started (prior to effectively running luci);
this can be disabled by setting this value to nonzero