unify systemtap and dtrace interfaces

First there were two probes that were defined but not used, perhaps dropped during the streams2 refactoring, I've added those back to `lib/net.js`.

Second it removes a bunch of redundancies between the dtrace and systemtap subsystems, which should make it easier to add/remove/change probes, but there's still an extra step for ETW.

I've also added a tapset which makes it prettier and easier to do tracing on systemtap

```
stap -I ~/.node/share/systemtap/tapset/ -e 'probe node_http_server_request { println(probestr); }'
http__server__request(remote=127.0.0.1, port=55750, method=GET, url=/wendell, fd=12)
http__server__request(remote=127.0.0.1, port=55751, method=GET, url=/wendell?limit=5&offset=5, fd=14)
http__server__request(remote=127.0.0.1, port=55752, method=GET, url=/allison?limit=5&offset=5, fd=15)
http__server__request(remote=127.0.0.1, port=55753, method=GET, url=/allison?limit=5&offset=5, fd=16)
http__server__request(remote=127.0.0.1, port=55754, method=GET, url=/wendell?limit=5, fd=13)
http__server__request(remote=127.0.0.1, port=55755, method=GET, url=/allison?limit=5&offset=5, fd=14)
http__server__request(remote=127.0.0.1, port=55756, method=GET, url=/allison?limit=5&offset=5, fd=12)
http__server__request(remote=127.0.0.1, port=55757, method=GET, url=/allison?limit=5, fd=12)
http__server__request(remote=127.0.0.1, port=55758, method=GET, url=/uter?limit=5, fd=12)
```