[rfc] move activation to a helper process

[Havoc Pennington]

John (J5) Palmieri wrote:
>> Certainly the patch is (or, at least, can be) done in a way that's
>> compatible with 1.0 (e.g. presence of User key).
> 
> If it can avoid breaking 1.0 it will be adopted much faster.

One thing to be sure to do is be sure 1.0 ignores (or just warns about, 
then ignores) unknown keys in the .service file, then apps could support
new .service file features without giving up 1.0 compat.

The downside to this kind of permissive parsing is always that it 
doesn't print errors about typos (you type "Exc=foo" and it just 
silently doesn't work), so maybe logging a warning is a good idea.

Havoc