tvheadend: add new package

tvheadend: add new package

Tvheadend is a TV streaming server and recorder for Linux. Legacy
Makefile for tvheadend was used as a skeleton, but most of its parts
were rewritten or updated later.

Procd init script allows to set some configuration options (these are
commented out in default Uci config for documentation purposes) and is
written in a way that it puts only configuration that needs to be
persistent to main memory. EPG database which is large and updated
regularly is put to tmpfs (this can be disabled in config).

Signed-off-by: Jan Čermák <jan.cermak@nic.cz>