MDEV-25359 : Improve mariabackup SST script compliance with native MariaDB SSL practices and configuration.

1. Pass joiner's authentication information to donor together with address
   in State Transfer Request. This allows joiner to authenticate donor on
   connection. Previously joiner would accept data from anywhere.

2. Deprecate custom SSL configuration variables tca, tcert and tkey in favor
   of more familiar ssl-ca, ssl-cert and ssl-key. For backward compatibility
   tca, tcert and tkey are still supported.

3. Allow falling back to server-wide SSL configuration in [mysqld] if no SSL
   configuration is found in [sst] section of the config file.

4. Introduce ssl-mode variable in [sst] section that takes standard values
   and has following effects:
    - old-style SSL configuration present in [sst]: no effect
      otherwise:
    - ssl-mode=DISABLED or absent: retains old, backward compatible behavior
      and ignores any other SSL configuration
    - ssl-mode=VERIFY*: verify joiner's certificate and CN on donor,
                        verify donor's secret on joiner
                        (passed to donor via State Transfer Request)
                        BACKWARD INCOMPATIBLE BEHAVIOR
    - anything else enables new SSL configuration convetions but does not
      require verification

    ssl-mode should be set to VERIFY only in a fully upgraded cluster.

    Examples:

    [mysqld]
    ssl-cert=/path/to/cert
    ssl-key=/path/to/key
    ssl-ca=/path/to/ca

    [sst]

     -- server-wide SSL configuration is ignored, SST does not use SSL

    [mysqld]
    ssl-cert=/path/to/cert
    ssl-key=/path/to/key
    ssl-ca=/path/to/ca

    [sst]
    ssl-mode=REQUIRED

     -- use server-wide SSL configuration for SST but don't attempt to
        verify the peer identity

    [sst]
    ssl-cert=/path/to/cert
    ssl-key=/path/to/key
    ssl-ca=/path/to/ca
    ssl-mode=VERIFY_CA

     -- use SST-specific SSL configuration for SST and require verification
        on both sides
Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>