Support socat parametrization to perform SST

Currently, even though the buffer setting for socat was noted as useful in upstream before:
https://galeracluster.com/library/documentation/xtrabackup-options.html

The default and recommended utility is Socat, given that it allows for socket options, such as transfer buffer size. 

the transferfmt reads only "nc" vs "socat" options, and the additional params are ignored, as the command is hard-coded later in the script. I think socat params should be optionable in [SST] section as well, especially large buffer.

For instance, to SST without encryption, you'd need to modify this code part: