Fixes for server header

[Natrinicle]

Allow address to be undef so that rsyncd can listen on all interfaces
useful for IPv4/IPv6 dual stack boxes where you want the daemon to
listen on both simultaneously. By setting address to :: the server
was only listening on IPv6 but setting it to 0.0.0.0 it was only
listening on IPv4. Leaving it out of the config makes the daemon
listen on all interfaces at startup.

[TheMeier]

Uhm istn't there a way to specify multiple listen addresses in rsyncd.conf?
If not i guess the best way to go forward would be to use systemd socket activations.

Anyway there might propably a valid use case in simply ommiting the address config all together

Could update the PR with some tests for the changes made?

[Natrinicle]

I couldn't find a way to specify multiple listen addresses in rsyncd.conf but perhaps there is a way.

I have changed the test for defaults to check that the address line isn't added to the config. The override is already there by testing for 10.0.0.42.

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