rsync: support for version 3.2.4

[oliver-sanders]

Cylc's internal rsync usage has compatibility issues with the recent 3.2.4 version, see:

• https://cylc.discourse.group/t/wrong-rsync-command/526/19
• https://download.samba.org/pub/rsync/NEWS#3.2.4

Either remove the need for escaping (preferred) OR there is an environment variable (RSYNC_OLD_ARGS) which can restore compatibility. We should be able to set this for our internal rsync uses (and only our internal uses).

Pull requests welcome!