Add ability to set VM options from config file in Debian

From a comment in my blog post:
here comes a little suggestion/feature request concerning the debian /etc/init.d/openfire script.
as probably many people out there (like me) use openfire on one of those crappy (but cheeaaap) virtual dedicated systems (vps) with limited memory, it would be cool if we could store somewhere in the config the maximum memory (without manuall editing the init script and putting the -Xmx100m parameter in the DAEMON-OPTS line.

if the init script would read this setting from the /etc/openfire config somewhere, it would be safe to just overwrite the init file with every update (to get the changes there as well - if there are any)

While Debian doesn't have something standard like redhat based linuxes (/etc/sysconfnig), that's a good idea, just put it in /etc/openfire somewhere.