Fix compile for Carbon

Protect the usage of qt_mac_loadMenuNib so it is only used on Cocoa as
this call is not used in Carbon code.

Change-Id: I93b9b513dce934680e1d5e1fb26bf2a64e25c764
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Ensure the webftp example works with newer versions of webkit.

The code currently has a bug that it will return the incorrect number
for bytesAvailable() since it doesn't take account of the QIODevice
buffer. This fails when peek() is called (which is used by newer
webkits). See QTBUG-27469 for an example of the problem.

Note this example does not exist in qt5 so this is not a backport.

Change-Id: Ic61248d0f36db6a6a65d0c48a8f40346dc7c3cb6
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>