Implement kmesgbuffer on macos and linux

Fixed the implementation of the kmesgbuffer() call on linux and then macos.

• It's a simple string interface at the moment. I thought it could have a different interface with a struct and vector of lines, lines parsed into fields etc, but didn't bother doing that in first implementation as I'm not sure anyone will even use this. Submit an enhancement request issue (or PR!) if you want that....