After switching to the new mariabackup interface (instead of
the outdated innobackupex interface, which is supported for
compatibility), we need to explicitly pass a path to the datadir
directory as a parameter, since in the new interface the value
of this option is not automatically set in such a way that it
always matches the SST/IST logic. This commit adds passing this
option as an explicit parameter to mariabackup. This commit also
removed unnecessary options that are not used and not supported
by mariabackup.

Also, numerous flaws in the common wsrep_sst_common script have
been fixed:

 1) There are many bash-specific constructs in the script that
    may not be supported by other interpreters, which can lead
    to the most unexpected errors during SST, because failures
    in the interpretation of bash-specific constructs lead to
    incorrect parsing of arguments;
 2) There is parse_cnf() function which is often called by other
    scripts for the "mysqld" or "--mysqld" group, but it does not
    take into account the default group suffix, which leads to
    reading values only from the default group, which then leads
    to errors due to reading the default values instead of the
    values for a specific group;
 3) Some options such as --user, --innodb-data-home-dir or --datadir
    are not removed from the --mysqld-args list, although they are
    processed inside scripts (and passing of these options funther
    may cause problems for mariabackup);
 4) If an argument that the script understands is present in
    the --mysqld-args list twice, then this causes SST to fail,
    instead of reading the most recent value;
 5) The "--host" parameter is technically still supported among
    the arguments of the SST scripts, but in reality scripts do not
    work with it as expected, especially if it has an IPv6 address;
 6) If the port number is absent in the --address parameter value,
    but the port number is explicitly passed through the --port
    argument, then the scripts for mariabackup and xtrabackup-v2
    fail;
 7) If a new address interface is used (with the --address parameter),
    then automatic default port substitution is not performed, although
    it is supported for the legacy --host/--port interface.
 8) If there are spaces in the parameter values after --mysqld_args,
    then their further transfer does not occur correctly, which
    causes mariabackup to fail during SST - the space splits
    the argument in such a way that it breaks the parsing of the
    following parameters;
 9) If most of the parameters that are names or paths to the files
    or directories contain spaces, then SST scripts fail in an
    unpredictable way due to incorrect variable substitutions;
10) If the --log-bin option is passed among the arguments of myqlds
    (--mysqld-args) without a parameter, and the --binlog option
    is not specified, then the script cannot substitute the default
    name for binlog and cannot construct binlog name using the
    --log-basename argument (which is against server specifications);
11) Tail slashes are not removed from the directory names, which,
    upon further substitution, leads to the appearance of a double
    slash in the file paths;
12) The explicit --binlog parameter (which is now always transmitted
    from the server side) and the "hidden" --log-bin parameter in the
    list of arguments after --mysqld-args are perceived as two different
    parameters in different parts of the scripts, and if they are do not
    match for some reason, this will lead to failures during SST;

Also, all new changes from the 10.6 branch have been migrated here,
including the latest pull requests for authentication (only the part
that concerns SST scripts).

It also fixes dozens of other bugs in all SST scripts.