[mythtv] mythfilldatabase environment requirements

[Robin Gilks]

I'm somewhat stuck with using tv_grab_nz-py in conjunction with
mythfilldatabase.

tv_grab_nz-py works great from the command line but running it from
mythbackend as a scheduled mythfilldatabase activity causes problems which
may be related to my environment. If I can get some idea of the
expectations of the grabber/backend combo then perhaps I can fix it!!

I run the backend as the user 'mythtv'. This user has home directory of
/home/mythtv and a shell of /bin/false so as to prevent login attempts on
my ssh port. When I fire up the backend I export HOME and PATH environment
variables as /home/mythtv and /usr/bin:/usr/local/bin repectively and the
backend itself is quite happy with this.