desktop-wide env vars

[Seth Nickell <seth gnome org>]

It might be worthwhile having a standard way/place to document env vars
relevant to modules. They're rather difficult to find by hunting through
source code, and assuming they have a reason to exist, are probably
relevant to distros and the like packaging GNOME.

Another place that it would be useful to include this information is in
the Sysadmin guide. 

Particularly important to sysadmins are env vars relevant to running
GNOME and apps out of different prefixes. For example, we've now got
XDG_DATA_DIRS, and I can't even find the env var (if one exists) for
pointing bonobo-activation to alternate .server directories.

-Seth