Werk #7482: mk_mysql: More consistent naming of instances

The naming of different mysql instances has been simplified. The item names of the monitored instances are now given by the corresponding sockets (e.g. an instance using the socket "/var/run/mysql.sock" will be called "/var/run/mysql.sock"). If no socket can be determined, only one instance will be monitored, using the default name "mysql".

Previously as a third attempt the --user= option found in the output of ps -fww -C mysqld has been used as instance name. This is not longer considered.

The determination of the sockets themselves is unchanged: If no sockets are listed in the config file, the socket is determined by the --socket= option found in ps -fww -C mysqld.

If you are monitoring mysql instances that do not use the default item name "mysql", a rediscovery is required.

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