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. In addition,
one of the files in the galera_3nodes suite does not contain the
option "--bind-address=::" which 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
The second line of changes related to replacing xtrabackup with
mariabackup:
1) All unnecessary references to xtrabackup are removed from
the documentation, from some comments, from the control files
that are used to prepare the packages.
2) Made corrections of the tests from the galera_3nodes suite
that mentioned xtrabackup or the old (associated with xtrabackup)
version of innobackupex.
3) Fixed flaws in the galera_3nodes mtr suite control scripts,
because of which they could not work with mariabackup.
4) 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.
5) Fixed flaws in tests for rsync and mysqldump (for galera_3nodes
mtr tests suite). These tests were not performed successfully without
these fixes.
https://jira.mariadb.org/browse/MDEV-17835
