backup: use `rsync -z` conditionally

'-z' depends on internal zlib which 3.1.1 disables by default, and the
new '-zz' breaks with servers running rsync ≤ 3.1.0, so...
backup: it's -zz, not -z

'-z' is never available on clients with external zlib; '-zz' depends on
the server.
backup: always add '-z' again

Fixed in Arch FS#41024 <https://bugs.archlinux.org/task/41024>
put common functions in Nullroute::LDAP

I have some stuff in Cluenet::LDAP and UK::LDAP that ought to be shared,
not copypasted.