Moved chk_valid_btun() from module-dvbapi.c to oscam-chk.c

ECM requests are now passed if a betatunnel is specified but the original caid is missing in the corresponding user services.
Using cccam reshare_mode = 3 we aren't forced anymore to specify the original caid which had the side effect that an additional virtual SC was added to the share-list.

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@8577 4b0bc96b-bc66-0410-9d44-ebda105a78c1