Changes

config/sys.config: group sctp_{client,server} params into maps

A follow-up patch brings even more SCTP SockOpts related params.
Let's prepare for this by grouping the existing sctp_{client,server}
related variables into distinct maps and moving the socket options
into their own sub-map.  Keep support for legacy options, for the
sake of backwards compatibility.

This helps to avoid having two instances of each new parameter
(s1gw_ and mme_) and generally improves readability/editability
of the config file.

Change-Id: Ib573915fdeb3ce73a1f6110c19fd6b1d46e68a03
Related: SYS#7653