phase1: make 'rsync -4' worker-configurable

phase1: make 'rsync -4' worker-configurable

We set '-4' in rsync_defopts, asking rsync to "prefer" ipv4 whenever
possible on _all_ workers. This was historically done because some
workers had flaky ipv6 connectivity, however in the future the reverse
may be true, with worker having worse networking over ipv4 than ipv6.

This change introduces an 'rsync_ipv4' worker configuration option:
when set to a true value, the old rsync behavior (adding '-4') is used,
when unset or set to false, the extra rsync argument is not used.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>