Before, loading a module would be guarded by an optional platform
condition. This made it possible to avoid loading and evaluating a
module if it did not support the host platform.
Unfortunately, this made it impossible to share a single configuration
between GNU/Linux and Darwin hosts, which some wish to do.
This removes the conditional load and instead inserts host platform
assertions in the modules that are platform specific.
Fixes#1906
I also made some modifications to the systemd service to match the [AUR version](https://aur.archlinux.org/cgit/aur.git/tree/goimapnotify@.service?h=goimapnotify) of `goimapnotify`. In particular, restarting is useful in case a network failure causes `imapnotify` to exit - that shouldn't mean that it stops trying when the network comes back up.
