build: allow building with dtrace on osx
dtrace: pass more arguments to probes

OSX and other DTrace implementations don't support dereferencing
structs in probes. To accomodate that pass members from the struct as
arguments so that DTrace is useful on those systems.
dtrace: actually use the _handle.fd value

When using the DTrace/systemtap subsystems it would be helpful to
actually have an fd associated with the requests and responses.