[tvheadend] expand dvr directories to full hierarchy.
[tvheadend] remove missing files from recordings list.
[tvheadend] fix default webroot param.
[tvheadend] ensure channel count is not cleared after succesful sub

This is particularly apparent for already subscribed multiplexes
where the subscriptionStart message comes back very quickly (possibly
even before the response to subscribe). Thus meaning the channel
count has been setup by the point SendSubscribe() exits and was
subsequently cleared.
[tvheadend] bump to 1.6.18
Merge branch 'pvr.hts'. closes #149. closes #150. closes #153. closes #154. closes #155