This patch contains the port of the MDEV-18379 patch
for 10.2 branch, but also includes a number of changes
made within MDEV-17835, which are necessary for the
normal operation of tests that use IPv6:

1) Currently, the three-node mtr suite for Galera (galera_3nodes)
uses a separate IPv6 availability check using the "have_ipv6.inc"
file. This check duplicates a more accurate check at suite.pm
level, which can be used by including the file "check_ipv6.inc".
This patch removes this discrepancy between suites.

2) Fixed numerous bugs in the SST scripts and in the mtr test
files (galera_3nodes mtr suite) that prevented the use of Galera
with IPv6 addresses.

3) Fixed flaws in the galera_3nodes mtr suite control scripts,
because of which they could not work with mariabackup.

4) Fixed flaws in the rsync and mysqldump tests (for galera_3nodes
mtr tests suite). These tests were not performed successfully
without these fixes.

5) GAL-501 test in the galera_3nodes suite does not contain the
option "--bind-address=::" that is needed for the test to work
correctly with IPv6 (at least on some systems), since without
it the server will not wait for connections on the IPv6
interface.

https://jira.mariadb.org/browse/MDEV-18379
and partially https://jira.mariadb.org/browse/MDEV-17835