MDEV-21298: mariabackup doesn't read from the [mariadbd] and [mariadbd-X.Y]
server option groups from configuration files
MDEV-21301: mariabackup doesn't read [mariadb-backup] option group in
configuration file

All three issues require to change the same code, that is why their
fixes are joined in one commit.

The fix is in invoking load_defaults_or_exit() and handle_options() for
backup-specific groups separately from client-server groups to let the last
handle_options() call fail on unknown backup-specific options.

The order of options procesing is the following:
1) Load server groups and process server options, ignore unknown
options
2) Load client groups and process client options, ignore unknown
options
3) Load backup groups and process client-server options, exit on
unknown option
4) Process --mysqld-args command line options, ignore unknown options

New global flag my_handle_options_init_variables was added to have
ability to invoke handle_options() for the same allowed options set
several times without re-initialising previously set option values.

--password value destroying is moved from option processing callback to
mariabackup's handle_options() function to have ability to invoke server's
handle_options() several times for the same possible allowed options
set.

Galera invokes wsrep_sst_mariabackup.sh with mysqld command line
options to configure mariabackup as close to the server as possible.
It is not known what server options are supported by mariabackup when the
script is invoked. That is why new mariabackup option "--mysqld-args" is added,
all unknown options that follow this option will be silently ignored.

wsrep_sst_mariabackup.sh was also changed to:
- use "--mysqld-args" mariabackup option to pass mysqld options,
- remove deprecated innobackupex mode,
- remove unsupported mariabackup options:
    --encrypt
    --encrypt-key
    --rebuild-indexes
    --rebuild-threads