Backport: use backslashes for UNCs path

ShellExecute fails to open a share folder due to using '/' instead of '\'.

Windows API doesn't support forward slashes for extended-length path.
Extended-length path are path that start with a "\\?\" prefix. For example,
"\\?\c:\very_long_path\foo\bar.txt", or in the case of a UNC path that
would be "\\?\very_long_path\foo\bar.txt". [1]

[1] http://msdn.microsoft.com/en-us/library/aa365247%28VS.85%29.aspx#maxpath

(cherry-picked and adapted from qtbase/18c916517f3004d34482c20ed66bf09ec274d385)
Task-number: QTBUG-13359
Change-Id: I2f38ecb76d87b87dc1d40bd48c94c78550a6ca1f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>