multimedia/tvheadend: Fix 'internal grabbers' to work properly. Fix rclint.

* New port option for 'textproc/xmltv' (tvheadend's idea of "internal grabbers")
* A user login account for tvheadend is required to configure the tv_grab_* EPG grabber. Otherwise the grabber will not work.
* tvheadend's $PATH environment variable must include /usr/local/bin, otherwise tvheadend it will not see / detect the grabber executables. PATH= statement added to rc.d script.
* Fix rclint errors in rc.d script.