multimedia/tvheadend: fix two major problems of webui on FreeBSD
- very high CPU usage while streaming a recored program
- unable to stream the recorded program beyond 128MB.
Unlike Linux sendfile(), FreeBSD sendfile() requires an explicit
file offset, and return value must be checked to catch any error
occurred. (i.e., closed connection)
Obtained from: https://github.com/tvheadend/tvheadend/pull/1125
Submitted by: Jongsung Kim <jongsung.kim@gmail.com>