Change base directory for Retro

[Mako88]

This changes the base directory on Mac and Linux.

I'll create another pull request for setting missions, players, and demos directories independently after I code it and test it :)

[ryusei117]

I've been using these patches for a while now, applying them whenever I build executables - and I gotta say, having separate directories for Rebirth and Retro has been very useful. :-) Since the development versions of Rebirth have some settings that Retro doesn't have and vice versa, using separate directories prevents them from erasing each other's unique settings. If this pull request were merged, and some players still wanted Rebirth and Retro to use the same directories, they could simply create symbolic links named ~/.d*x-retro targeting ~/.d*x-rebirth (and the same for the sharepath directories).

There's one minor mistake in commit "Change name of executable to d*x-retro", though: on line 638 of file d1/SConstruct, the help text needs to be changed to show that the new default sharepath is /usr/local/share/games/d1x-retro. Easy fix. ;-)