Add SO_SNDBUF and SO_RCVBUF to socket options

Added functionality to set SO_SNDBUF and SO_RCVBUF
socket options on QAbstractSocket.

Task-number: QTBUG-34934
Change-Id: I2134fb462d43b9111c039cd7e7d36bd78eafd8bc
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Richard J. Moore <rich@kde.org>