FreeBSD syscalls: change two more warnings

FreeBSD syscalls: change two more warnings

sysctl gave me a hard time when I first started working on FreeBSD
but that warning should only be without -q

change the unhandled kenv warning to a VG_(unimplemented), which will
terminate Valgrind rather than just print a warning.

There are still a few aio warnings. Really they should be promoted
to some kind of fully fledged warning (maybe a core warning). I'm
not sure that it's worth the effort as I suspect that aio is not
much used.