Restore Darwin OS configuration

MacStadium are providing us with free Mac Mini hosting as part of their
FOSS support.  I'm about to set it up.  Let's have out-of-repo tuning in
place before I begin.

diff --git a/src/OS/unsupported/Makefile-Darwin b/src/OS/Makefile-Darwin
similarity index 100%
rename from src/OS/unsupported/Makefile-Darwin
rename to src/OS/Makefile-Darwin

diff --git a/src/OS/unsupported/os.h-Darwin b/src/OS/os.h-Darwin
similarity index 100%
rename from src/OS/unsupported/os.h-Darwin
rename to src/OS/os.h-Darwin