unison: fix escaping of arguments

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

(cherry picked from commit 92c682cd10)

Fixes #1500

modules/services/unison.nix

@@ -60,7 +60,7 @@ let
     };
   };
 
-  serialiseArg = key: val: "-${key}=${escapeShellArg val}";
+  serialiseArg = key: val: escapeShellArg "-${key}=${escape [ "=" ] val}";
 
   serialiseArgs = args: concatStringsSep " " (mapAttrsToList serialiseArg args);