Freebsd

Hi Nenad,

The gist of these changes is that I've moved system specific code out of %POSIX.reds so that it can be used by all POSIX systems. This has reduced code duplication a bit. Platform independent signal definitions has been added in %POSIX-signals.reds.

I'm much happier with this, but it's a lot of restructuring, so let me know if you want it differently. All tests have passed on FreeBSD.

Cheers!