QSettings: undo unintentional change of config dir on non-XDG platforms.

c99dfd8f631289 only meant to be able to switch to the test mode of
QStandardPaths, not to move the default dir on OS X, iOS, BB10 and Android.
So this commit restores it to the previous behavior, to avoid migration issues.
The use of XDG_CONFIG_HOME, defaulting to ~/.config, on OS X, is even documented
in the current QSettings documentation, even though these paths are non-standard
on OS X (granted, the use of ini-style config files isn't either).

Task-number: QTBUG-41461
Change-Id: I5eb610ff7ccbdaf6f955ef7f8f7c2658cbecbb86
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
prevent if_nametoindex being called with empty string

Calling if_nametoindex with an empty string will always return 0, but
on ARM linux platforms this call seems to be very expensive (~30ms),
adding a large overhead to calls such as QUdpSocket::writeDatagram()

Task-number: QTBUG-37092
Change-Id: Iad00867585d9534af1ddaee936dd4e4dc5e03611
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>