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 `ExecStart=` option of systemd must take arguments fully quoted.
That is,
"-sshargs=-i somekey"
and not
-ssargs="-i somekey"
Additionally, inside arguments passed to unison, `=` characters must
be quoted. After unquotation by systemd, one must have
-sshargs=-o Foo\=4
instead of
-sshargs=-o Foo=4
