Make socket descriptors qintptr.

Make socket descriptors qintptr.

Windows x64 uses 64 bits integer for sockets, to ensure compatibility we
should use ptr sized integers for our socket descriptors.

Task-number: QTBUG-19004
Change-Id: I4b56023874a4f1bad107c66c054fecfedde33d88
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>