Somewhat less ifdeffery, higher flexibility. Now there are 3 separate
config file resolvers for 3 platforms (unix, win, osx), and they can
still interact with each other somewhat. For example, OSX for now uses
most of Unix, but adds the OSX bundle path.
This can be extended to resolve very specific platform paths, such as
location of the desktop.
Most of the Unix specific code moves to path-unix.c.
The behavior should be the same - if not, it is likely a bug.
(cherry picked from commit d3a3cfe54c)
