SheepShaver on FreeBSD

It's working on FreeBSD i386 and amd64, with and without the JIT! Note that in FreeBSD 9.0 and higher one needs the sysctl "security.bsd.map_at_zero=1" to allow mapping low memory.

I don't anticipate this causing problems for other platforms, but I only checked on OS X so far. Please look over the changes to make sure they're sane :)