Revert "dvbapi-stapi: Stop using pthread_cancel() to kill stapi_read threads."

This reverts commit r8337.

Unfortunately (as expected) this causes problems because the threads
are not reaching the cleanup code. Anyway, it is safe to return to
the old code, because pthread_cancel is used in a code that can
never be compiled under Android.

Fixes ticket #3163.

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@8347 4b0bc96b-bc66-0410-9d44-ebda105a78c1