xtrabackup configuration parser

Hi,

There is an inconsistency in how MariaDB Server reads the configuration and xtrabackup reads the configuration. This results in to many quirks:

1. For xtrabackup --move-back or --copy-back, datadir needs to be present in /etc/my.cnf - no other configuration file works at all. This results in SST not working if it it not present or in /etc/my.cnf.d/server.cnf.
2. If innodb_log_files_in_group is present in 2 configuration files, creating the backup fails (some times, I assume when 1 value is lower then the other and the server is writing to higher redo log files then xtrabackup knows about).
3. For some operations (I don't remember which exactly), socket needs to be present in configuration file. This also needs to be present in /etc/my.cnf.

wlad mentioned it would be great if xtrabackup logs in to MariaDB and gets the configuration file from mariadb. I can imagine this works for most configurations but not all configurations (such as how to connect to the server).

The latter is needed for --copy-back and --move-back to work. To avoid double work I think it might be best to have the same mechanism of configuration file parsing in xtrabackup as MariaDB has. We can also make the target data directory a required argument when --move-back is used, and only read configurations from the running server.

Cheers,