new api to set the environment of activated services

[Ray Strode]

So I was look at:

http://bugzilla.gnome.org/show_bug.cgi?id=360475

and discussion led to the conclusion we should add some method for
setting the environment of session bus activated clients to help the
session manager/ bus daemon ordering problem mentioned on the bug
report.

I think something like this patch:

http://people.freedesktop.org/~halfline/activation-environment.patch

should work.  Thoughts?

--Ray