Split reading/writing preferences into OS specific files

This adds tested code for Linux and Mac OS, implementing the api that
Linus suggested.

The Windows code was moved into its own file, but hasn't even been compile
tested, yet.

In order to have just one interface to set or get a preference value we
encode TRUE as (void *) 1 and FALSE as NULL. This works consistently on
all platforms and regardless of whether we have 32 or 64 bit.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>