Debian package : easier way to change user renderd is run with

Maybe there was a reason under debian to force renderd to run with www-data, but in some other scenari cases, it might be interesting to run it with another user.

This pull request should not change much the current behaviour (unless you want to run renderd with another user but forcing it's run to the www-data user) and adds the possibility to add :
RUNASUSER=whatever 
in /etc/default/renderd
to make renderd run as "whatever"
(default is www-data)