Initial support for MacOS.

This will soon be complemented with MacOS specific source code files and
INSTALL.MacOS.

I (Andy) have decided to get rid of a number of #include <sys/types.h>.
I've verified it's ok (both by examining /usr/include/*.h and compiling)
on a number of Unix platforms. Unfortunately I don't have Windows box
to verify this on. I really appreciate if somebody could try to compile
it and contact me a.s.a.p. in case a problem occurs.

Submitted by: Roy Wood <roy@centricsystems.ca>
Reviewed by: Andy Polyakov <appro@fy.chalmers.se>