NSUser and paths generally

[Sheldon Gill]

It seems that NSUser.m was originally created by Andrew McCallum for a new 
class that handled login related functions.

However, it mainly contains the functions from "NSPathUtilities.h". I propose 
moving those functions to a new file "NSPathUtilities.m" where they would 
seem more at home.

I also propose making some modifications to the path handling routines:
* Initialise the various root directories from a plist in /etc/GNUstep if it 
exists, over-riding environment variables. This will allow system 
administrators to enforce system file heirarchy policy.
It'll also mean that, on such systems, you don't need to source GNUstep.sh!
* Add support for traditional locations for applications and libraries:
    NSApplicationDirectory   /bin /usr/bin
    GSLibrariesDirectory      /lib  /usr/lib
    NSAdminApplications    /sbin /usr/bin
These will be last in the returned array.
* Extend support for more parts of the heirarchy like:
    Fonts, Palattes, ColorPanels, Services, etc

What I wish to work towards is making GNUstep highly path layout agnostic and 
consolidate it's file dependancies.

This will ease packaging GNUstep for different platforms. I'm particularly 
thinking of Win32 but Debian will also benefit quite immediately. Other 
things to be done along these lines is extending the defaults system to allow 
for installation defaults. This would allow a system administrator to set up 
an environment for all users quite easily.