Changes

sctp_{client,server}: set sndbuf/recbuf explicitly

For some reason, Erlang/OTP does overwrite the OS-default
SO_SNDBUF/SO_RCVBUF values even if the respective socket options
are not given to gen_sctp:open/N explicitly.  Even worse, the
receive buffer size is set to a very low value (1024), resulting
in major performance issues when dealing with large packets.

Set both sndbuf/recbuf to 65536 explicitly - this value is used
for the sndbuf by default.  Allow tuning sndbuf/recbuf via the
environment variables.

Change-Id: I66f4a0f6a2441a3c8a4dbd266da8e44a3860b993
Related: SYS#7288