I've wanted to fix up the exim service given Ludo's comments on my last
few patches (see #25789), so here are some patches to introduce a
mail-aliases-service-type, to change exim-service-type to use it
(instead of doing its own thing), and to add a system test for
exim-service-type.

I haven't yet written the updates necessary to the documentation, but I
thought I'd send these now to get feedback before I do the work to fix
up the docs.