Fix building on FreeBSD and update the documentation

Mostly I just had to add FreeBSD to the list of supported operating systems in various places. In one place I had to change the path to find the path of the executable because it's different in FreeBSD.

Unfortunately in FreeBSD the only easy way (see https://stackoverflow.com/a/1024937/428740) to get this information programtically requires configuring procfs which I have documented. I searched the code and it's only used in one place if you use rostrum.

(this is a port from BCHU)