Make Symbian follow Windows code in temporary path generation

Make Symbian follow Windows code in temporary path generation

On the one hand, we stop using OpenC here. On the other, we no longer
use an atomic create and obtain file handle API -- just as we don't on
Windows yet.

This is a stepping stone to removing back and forth conversions of path
names when generating unique names and also towards the use of native
APIs for creating and obtaining a file handle atomically.

Reviewed-by: Gareth Stockwell
Reviewed-by: Shane Kearns
(cherry picked from commit 63bb67d3107b03f399cddf4c9cca9c7eb347b62d)

Change-Id: I97b3b6179dff053807acc8d4469fdf57f57f68a6
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>