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
The ExecStartPost command is currently started when the mbsync is
invoked succesfully. However, we typically want to run something like
'mu index' or 'notmuch new' after mbsync completes. This changes the
unit type to oneshot, so that the ExecStartPost command is run after
mbsync finishes succesfully.
