path handling changes to try out ...

[Richard Frith-Macdonald]

Several little changes ...

1. the NSSearchPathForDirectoriesInDomains() function now returns
the value specified by the GNUSTEP_USER_ROOT environment variable
rather than the unix home directory.

2. The location of the defaults database is now determined by the
path specified in GNUSTEP_DEFAULTS_ROOT, or if that is undefined
by GNUSTEP_USER_ROOT.

3. The defaults database is moved to the Defaults subdirectory of
the defaults root.

4. Changing the user of a process by calling GSSetUserName() now
alters the paths returned by NSSearchPathForDirectoriesInDomains()
as well as the path use to access the defaults databas ... so code
designed to operate under those circumstances should not cache
those paths.

Thanks to Nicola for the idea that gave me the clue to doing this -
The GNUSTEP_DEFAULTS_ROOT and GNUSTEP_USER_ROOT variables are used
to construct templates into which we substitute the NSHomeDirectory()
and NSUserName() values when we change users ... this is not perfect
as users may have set their own values for those variables, but it's
an improvement.

Actually, I wonder how much use all these environment variables are,
and whether it would be better to fix them at the point where the make
package is configured instead of permitting each user to redefine them
at will?

I also wonder how much use the GNUSTEP_PATHPREFIX_LIST environment
variable is ... it's really only partially supported right now.