Allow wsrep_sst_rsync to pass options to rsync binary via [sst] settings

Currently the wsrep_sst_rsync script is symlinked to wsrep_sst_rsync_wan, with the script checking which name it was called by, and adding the --whole-file when not seeing wan in the $0 command name variable.

This is sort of magic, and not documented beyond having a minimal man page just saying

WSREP_SST_RSYNC_WAN(1)         MariaDB Database System        WSREP_SST_RSYNC_WAN(1)

NAME

       wsrep_sst_rsync_wan - rsync_wan (rsync with delta transfers)-based state

       snapshot transfer

DESCRIPTION

       Use: rsync_wan-based state snapshot transfer.

       For more information, please refer to the MariaDB Knowledge Base, available

       online at https://mariadb.com/kb/

MariaDB 10.6                         15 May 2020              WSREP_SST_RSYNC_WAN(1)

I'd suggest to have a [sst] parameter like e.g. rsync_wan, and maybe also a generic rsync_options instead to be able to have all configuration settings in one place.

And of course: document it properly ...