Added support in MacPorts base to set PATH and MANPATH automatically in Leopard

[James Berry]

I just checked in some code (r31491:31492) that supplies and installs  
spec files for the /etc/paths.d  and /etc/manpaths.d directories if  
they are present, as they are on Leopard.

The contents of these files are automatically added to the users PATHS  
and MANPATHS environment variables, and should mean that we don't need  
to configure the user's profile under Leopard. Note that support for  
this is though path_helper(8).

Hopefully this will make future configuration less painful. Comments  
welcome.

James